# strongly connected components vs connected components

The time complexity of this algorithm is … The Strongly Connected Components (SCC) algorithm finds maximal sets of connected nodes in a directed graph. Now the only problem left is how to find some node in the sink Strongly Connected Component of the condensed component graph. The strongly connected components are identified by the different shaded areas. These examples are extracted from open source projects. Now, a $$DAG$$ has the property that there is at least one node with no incoming edges and at least one node with no outgoing edges. And now the order in which $$DFS$$ on the new sinks needs to be done, is known. Assignment 4, Standford Algorithms MOOC #1. The strongly connected components form an acyclic component graph that represents the deep structure of the original graph. Case 2: When $$DFS$$ first discovers a node in $$C'$$: Now, no node of $$C$$ has been discovered yet. First define a Condensed Component Graph as a graph with $$\le V$$ nodes and $$\le E$$ edges, in which every node is a Strongly Connected Component and there is an edge from $$C$$ to $$C'$$, where $$C$$ and $$C'$$ are Strongly Connected Components, if there is an edge from any node of $$C$$ to any node of $$C'$$. Let G=(V, E) be a directed graph where V is the set of vertices and E the set of edges. The sheer number of nodes combined with the recursive solution that was utilized caused a stack overflow to occur. Hence it is a separate strongly connected component. Is a single undirected edge be called a Strongly connected component? Now, to find the other Strongly Connected Components, a similar process must be applied on the next element(that is $$2$$), only if it has not already been a part of some previous Strongly Connected Component(here, the Strongly Connected Component of $$1$$). The first linear-time algorithm for strongly connected components is due … These mutually connected regions represent the core structure of the clusters. Therefore for this case, the finish time of some node of $$C$$ will always be higher than finish time of all nodes of $$C'$$. So, how to find the strongly connected component which includes node $$1$$? A cyclic graph is formed by connecting all the vertex to the closest components. This can be done with a stack, when some $$DFS$$ finishes put the source vertex on the stack. Hence it violates the laws of Strongly connected components. Note that the Strongly Connected Component's of the reversed graph will be same as the Strongly Connected Components of the original graph. Finding-Strongly-Connected-Components. 104 On finding the strongly connected components in a … Now observe that on the cycle, every strongly connected component can reach every other strongly connected component via a directed path, which in turn means that every node on the cycle can reach every other node in the cycle, because in a strongly connected component every node can be reached from any other node of the component. For example, there are 3 SCCs in the following graph. To prove it, assume the contradictory that is it is not a $$DAG$$, and there is a cycle. Strongly connected component, a related concept for directed graphs; Biconnected component; Modular decomposition, for a proper generalization of components on undirected graphs; Connected-component labeling, a basic technique in computer image analysis based on components of graphs; Percolation theory, a theory describing the behavior of components in random subgraphs of … From the DFS tree, strongly connected components are found. After all these steps, the list has the following property: every element can reach $$ELE$$, and $$ELE$$ can reach every element via a directed path. Then, if node $$2$$ is not included in the strongly connected component of node $$1$$, similar process which will be outlined below can be used for node $$2$$, else the process moves on to node $$3$$ and so on. For example: Let us take the graph below. If the graph is not connected the graph can be broken down into Connected Components. A strongly connected component in a directed graph refers to a maximal subgraph where there exists a path between any two vertices in the subgraph. 2. HackerEarth uses the information that you provide to contact you about relevant content, products, and services. Equivalence class are called strongly-connected components. Case 1: When $$DFS$$ first discovers a node in $$C$$: Now at some time during the $$DFS$$, nodes of $$C'$$ will start getting discovered(because there is an edge from $$C$$ to $$C'$$), then all nodes of $$C'$$ will be discovered and their $$DFS$$ will be finished in sometime (Why? Many people in these groups generally like some common pages, or play common games. 1) Create an empty stack ‘S’ and do DFS traversal of a graph. Generally speaking, the connected components of the graph correspond to different classes of objects. Strongly connected components can be found one by one, that is first the strongly connected component including node $$1$$ is found. Following is detailed Kosaraju’s algorithm. $$DFS$$ of $$C'$$ will visit every node of $$C'$$ and maybe more of other Strongly Connected Component's if there is an edge from $$C'$$ to that Strongly Connected Component. Firstly a directed graph is definitely not an undirected graph but a subset of it. 19, Nov 19. The first linear-time algorithm for strongly Complete reference to competitive programming. 7.8 Strong Component Decomposing a directed graph into its strongly connected components is a classic application of depth-first search. $$2)$$ Reverse the original graph, it can be done efficiently if data structure used to store the graph is an adjacency list. For example, there are 3 SCCs in the following graph. Lets assume a has the highest finish time, and so if … The condensed component graph can be reversed, then all the sources will become sinks and all the sinks will become sources. Defining Strongly Connected Component Mathematically: Strongly connected implies that both directed paths exist. A strongly connected component (SCC) of a directed graph is a maximal strongly connected subgraph. This is because, in the above diagram, component 1–2–3 can reach any vertex (out of 1,2 and 3) starting from any vertex in the component. Definitely, you do. Upon performing the first DFS with scc1 as the source, we get the following scenario: Upon reversing the graph and performing DFS again with scc2 as the source, we get the following scenario: We infer that after both the DFS passes, the strongly connected components are clustered together. In social networks, a group of people are generally strongly connected (For example, students of a class or any other common place). 0 answers. It should also check if element at index $$IND+1$$ has a directed path to those vertices. The weakly connected components are found by a simple breadth-first search. In slightly more theoretical terms, an SCC is a strongly connected subgraph of some larger graph G. So that graph above has four SCCs. Is acyclic graph have strongly connected components the same as connected components? In other words, topological sorting(a linear arrangement of nodes in which edges go from left to right) of the condensed component graph can be done, and then some node in the leftmost Strongly Connected Component will have higher finishing time than all nodes in the Strongly Connected Component's to the right in the topological sorting. This means that strongly connected graphs are a subset of unilaterally … 94 """Returns list of strongly connected components in G. 95 Uses Tarjan's algorithm with Nuutila's modifications. The problem of finding connected components is at the heart of many graph application. A1. But now if we try to add 4 to the above component and make 1–2–3–4 as a single component, it is observed that we cannot reach any vertex from any vertex like suppose if we start from 4, we cannot connect to 1 or 2 or 3. But definitely can have the same number of components when undirected only. The simplified version of the graph in Figure 31 is … A Strongly Connected Component is the smallest section of a graph in which you can reach, from one vertex, any other vertex that is also inside that section. So, initially all nodes from $$1$$ to $$N$$ are in the list. This is because it was already proved that an edge from $$C$$ to $$C'$$ in the original condensed component graph means that finish time of some node of $$C$$ is always higher than finish time of all nodes of $$C'$$. Since edges are reversed, $$DFS$$ from the node with highest finishing time, will visit only its own Strongly Connected Component. Q1. 20, Aug 14. Your Task: You don't need to read input or print anything. Thus, may not have 1 strongly connected component. component_distribution creates a histogram for the maximal connected component sizes. For each test case in a new line print, the Strongly connected component of a graph where each member of a strongly connected component is separated by a comma (",") and each strongly connected components is separated by a new line. Depth-first search and linear graph algorithms. Parameters: G (NetworkX Graph) – An directed graph. Let there be a list which contains all nodes, these nodes will be deleted one by one once it is sure that the particular node does not belong to the strongly connected component of node $$1$$. A4. Check if there exists a connected graph that satisfies the given conditions. We can find all strongly connected components in O(V+E) time using Kosaraju’s algorithm. You can vote up the ones you like or vote down the ones you don't like, and go to the original project or source file by following the links above each example. Well, a strongly connected component is a subset of connected components. It has two strongly connected components scc1 and scc2. If you get anything else. It is also important to remember the distinction between strongly connected and unilaterally connected. If I go to node 2, I can never go to any other node, and then back to … The property is that the finish time of $$DFS$$ of some node in $$C$$ will be always higher than the finish time of all nodes of $$C'$$. Well, a strongly connected component is a subset of connected components. Decomposing a directed graph into its strongly connected components is a classic application of depth-first search. Thus the number of strongly connected componets=number of vertices=7, Similarly, the number of connected componets=7. if A to B vertices are connected by an edge then B to A must also be present. It can be proved that the Condensed Component Graph will be a Directed Acyclic Graph($$DAG$$). Well not actually. Tarjan's Algorithm to find Strongly Connected Components. The strongly connected components of the above graph are: Strongly connected components Q2. Kosaraju's Linear time algorithm to find Strongly Connected Components: This algorithm just does $$DFS$$ twice, and has a lot better complexity $$O(V+E)$$, than the brute force approach. So at each step any node of Sink should be known. It requires only one DFS traversal to implement this algorithm. If not, $$OtherElement$$ can be safely deleted from the list. Then, if node $$2$$ is not included in the strongly connected component of node $$1$$, similar process which will be outlined below can be used for node $$2$$, else the process moves on to node $$3$$ and so on. 101 SIAM Journal of Computing 1(2) :146-160. Strongly Connected Components (SCC) The strongly connected components (SCC) of a directed graph are its maximal strongly connected subgraphs. Now let’s observe that on the cycle, every strongly connected component can reach every other strongly connected component via a directed path, which in turn means that every node on the cycle can reach every other node in the cycle, because in a strongly connected component every node can be reached from any other node of the component. A strongly connected component (SCC) of a directed graph is a maximal strongly connected subgraph. Now, removing the sink also results in a $$DAG$$, with maybe another sink. So the above process can be repeated until all Strongly Connected Component's are discovered. A directed graph is strongly connected if there is a path between all pairs of vertices. Thus definitely connected components have only 1 component but we cannot reach any vertex from any other vertex only if directed. Queries to check if vertices X and Y are in the same … But what are strongly connected components? Call the above $$2$$ nodes as Source and Sink nodes. A set is considered a strongly connected component if there is a directed path between each pair of nodes within the set. If not, such nodes can be deleted from the list. This process needs to check whether elements at indices $$IND+2,...,LEN$$ have a directed path to element at index $$IND+1$$. In the end, list will contain a Strongly Connected Component that includes node $$1$$. … Well, I was just kidding. Using DFS traversal we can find DFS tree of the forest. The time complexity of the above algorithm is $$O(V^{3})$$. A strongly connected component (SCC) of a coordinated chart is a maximal firmly associated subgraph. Similar to connected components, a directed graph can be broken down into Strongly Connected Components. The option is pretty clear though. 96 Nonrecursive version of algorithm. #Algorithms #DFS How to find if a directed graph G is strongly connected using DFS in one pass? We can find all strongly connected components in O (V+E) time using Kosaraju’s algorithm. The following are 30 code examples for showing how to use networkx.strongly_connected_components(). One of nodes a, b, or c will have the highest finish times. The only difference is that in connected components we can reach any vertex from any vertex, but in Strongly connected components we need to have a two-way connection system i.e. Generate a sorted list of strongly connected components, largest first. A password reset link will be sent to the following email id, HackerEarth’s Privacy Policy and Terms of Service. Initial graph. Unfortunately, distances in RGB colour space do not reflect what … Rahul’s teacher asks him to apply DFS on a given graph of 7 vertices. So, for example, the graph that we looked at has five strongly connected components. 22, Apr 19. For each test case in a new line output will an integer denoting the no of strongly connected components present in the graph. How to find Strongly connected components and weakly connected components in the given graph? This should be done efficiently. Every single node is its own SCC. Else drop in our comment box, the part you are not comfortable with. But the connected components are not the same. The problem of finding connected components is at the heart of many graph application. 102 103 E. Nuutila and E. Soisalon-Soinen (1994). But, why are the strongly connected components not same as connected components. Now a $$DFS$$ can be done from the next valid node(valid means which is not visited yet, in previous $$DFSs$$) which has the next highest finishing time. So when the graph is reversed, sink will be that Strongly Connected Component in which there is a node with the highest finishing time. If we reverse the directions of all arcs in a graph, the new graph has the same set of strongly connected components as the original graph. Figure 31: A Directed Graph with Three Strongly Connected Components ¶ Once the strongly connected components have been identified we can show a simplified view of the graph by combining all the vertices in one strongly connected component into a single larger vertex. The strong components are the maximal strongly connected subgraphs Connected Components Strongly connected graph A directed graph is called strongly connected if for every pair of vertices u and v there is a path from u to v and a path from v to u. Note that "maximal" means that the set S is maximal, i.e., no more vertices can be added to S and still guarantee the mutual reachability property. Try doing again. Let’s have a look into this through an image. For instance, there are three SCCs in the accompanying diagram. In the mathematical theory of directed graphs, a graph is said to be strongly connected if every vertex is reachable from every other vertex. Q3. Q4. So to do this, a similar process to the above mentioned is done on the next element(at next index $$IND+1$$) of the list. Colours in our input image are represented in RGB colour space; that is each pixel is represented as three numbers corresponding to a red, green and blue value.In order to measure the similarity of a pair of colours the “ distance ” between the colours in the colour space can be measured. Observe that now any node of $$C$$ will never be discovered because there is no edge from $$C'$$ to $$C$$. The algorithm in steps can be described as below: $$1)$$ Do a $$DFS$$ on the original graph, keeping track of the finish times of each node. Rahul on doing so came up with the following conclusion: a) Each vertex has the same in-degree and out-degree sequence. Then which one of the following graphs has the same strongly connected components as G ? $$3)$$ Do $$DFS$$ on the reversed graph, with the source vertex as the vertex on top of the stack. The order is that of decreasing finishing times in the $$DFS$$ of the original graph. The Present Future of User Interface Development, Part 2: Build This Cool Dropdown Menu With React, React Router and CSS, Winds — An in Depth Tutorial on Making Your First Contribution to Open-Source Software, How to Write Test Cases for React Hooks From Scratch, Understanding The Web History API in JavaScript, How To Highlight Markdown Code With Remarkable. Check if a directed graph is connected or not. Now a $$DFS$$ can be done on the new sinks, which will again lead to finding Strongly Connected Components. The strongly connected components of an arbitrary directed graph form a partition into subgraphs that are themselves strongly connected. Weakly Prime Numbers. In DFS traversal, after calling recursive DFS for adjacent … The strongly connected components are implemented by two consecutive depth-first searches. Our empirical analysis and experimental results present the rationale behind our solution and validate the goodness of the clusters against the state of the art high … Connectivity in an undirected graph means that every vertex can reach every other vertex via any path. Similarly, if we connect 5 we cannot reach 1,2,3 or 4 from it hence it is a single and a separated component. In this way all Strongly Connected Component's will be found. Proof: There are $$2$$ cases, when $$DFS$$ first discovers either a node in $$C$$ or a node in $$C'$$. 20, Jun 20. So if there is a cycle, the cycle can be replaced with a single node because all the Strongly Connected Components on that cycle will form one Strongly Connected Component. Strong Connectivity applies only to directed graphs. Basic/Brute Force method to find Strongly Connected Components: Strongly connected components can be found one by one, that is first the strongly connected component including node $$1$$ is found. The complexity of the above algorithm is $$O(V+E)$$, and it only requires $$2 DFSs$$. discrete-mathematics; graph-theory; 0 votes. Let length of list be $$LEN$$, current index be $$IND$$ and the element at current index $$ELE$$. G is strongly connected if it has one strongly-connected component, i.e. Therefore, the Condensed Component Graph will be a $$DAG$$. It is applicable only on a directed graph. A directed graph is strongly connected if there is a directed path from any vertex to every other vertex. Now a property can be proven for any two nodes $$C$$ and $$C'$$ of the Condensed Component Graph that share an edge, that is let $$C \rightarrow C'$$ be an edge. If any more nodes remain unvisited, this means there are more Strongly Connected Component's, so pop vertices from top of the stack until a valid unvisited node is found. Strongly connected components are found through DFS only since here in a single undirected edge we can reach any vertex from any vertex so definitely it is strongly connected. But the elements of this list may or may not form a strongly connected component, because it is not confirmed that there is a path from other vertices in the list excluding $$ELE$$ to the all other vertices of the list excluding $$ELE$$. Now for each of the elements at index $$IND+1,...,LEN$$, assume the element is $$OtherElement$$, it can be checked if there is a directed path from $$OtherElement$$ to $$ELE$$ by a single $$O(V+E)$$ $$DFS$$, and if there is a directed path from $$ELE$$ to $$OtherElement$$, again by a single $$O(V+E)$$ $$DFS$$. When $$DFS$$ finishes, all nodes visited will form one Strongly Connected Component. Typically, the distance measured is the Euclidean distance. You may check out the related API usage on the … The SCC algorithms can be used to find … So clearly finish time of some node(in this case all) of $$C$$, will be higher than the finish time of all nodes of $$C'$$. This is same as connectivity in an undirected graph, the only difference being strong connectivity applies to directed graphs and there should be directed paths instead of just paths. Generate nodes in strongly connected components of graph. It is often used early in a graph analysis process to help us get an idea of how our graph is structured. H and I you can get from one to … SCC detection which decomposes a given directed graph into a set of disjoint SCCs is widely used in many graph alanytics applications, including web and social network analysis [16], formal veri•cation [12], reinforcement learning [15], mesh re•nement [22], … We care about your data privacy. Your task is to complete the function kosaraju() which takes the number of vertices V and adjacency list of the graph as inputs and returns an integer denoting the number of strongly connected components in the given graph. if every vertex is reachable from every other vertex. Now one by one, the process keeps on deleting elements that must not be there in the Strongly Connected Component of $$1$$. After you can get it all around around there, but there's no way to get from it to anything else. 187 views. But most importantly the statement doesn’t say that we need to have a direct path from A to B and B to A. 16, May 13. So, if there is an edge from $$C$$ to $$C'$$ in the condensed component graph, the finish time of some node of $$C$$ will be higher than finish time of all nodes of $$C'$$. On finding the strongly connected components ( SCC ) of a directed path to those vertices vertices... $2$ $can be safely deleted from the DFS tree, strongly connected components in G. 95 Tarjan. It violates the laws of strongly connected subgraph Decomposing a directed graph is not a$,! … Generate nodes in a graph analysis process to help us get an idea of how graph. Computing 1 ( 2 ):146-160 time complexity of the reversed graph will be same as connected is... And so on V^ { 3 } )  1  N $... References: 99 100 R. Tarjan ( 1972 ) IND+1$ $DFS$ $DFS$ $came..., with maybe another sink improve this answer | follow | edited 21! Looked at has five strongly connected component which includes node$ $DFS$ $feel free to ask nodes... The part you are not comfortable with components as G call the above algorithm is$. Path from any other vertex only if directed means that every vertex reach. The set another sink time, and there is a cycle to remember the distinction between strongly connected in... Maximal sets of connected nodes in a directed graph vertices and E the set of edges common.! When  can be done with a stack overflow to occur have a look into this an... Step any node of sink should be known from the list any node sink. '' '' Returns list of strongly connected components is a path between all pairs of vertices the order which! Component of the reversed graph will be on top of the original.. Assume the contradictory that is it is often used early in a directed graph is strongly component! Be sent to the closest components with the recursive solution that was utilized caused a stack, when $! Policy and Terms of Service largest first$ $has a directed graph the information you. Graph analysis process to help us get an idea of how our graph formed. Satisfies the given graph via any path comment box, the graph to! Print anything 7 vertices of depth-first search using Kosaraju ’ s have a look this! … in a directed graph these groups generally like some common pages, or play common games of all unvisited... Was utilized caused a stack, when some$ $OtherElement$ $) can! To 100+ Tutorials and Practice Problems Start now lets assume a has the same connected... Examples for showing how to use networkx.strongly_connected_components ( ) in my example there., products, and services highest finishing time of all currently unvisited nodes DFS in one pass distance measured the! Of graph v is the set maybe another sink often used early in a directed graph into its strongly components... 103 E. Nuutila and E. Soisalon-Soinen ( 1994 ) vertices=7, similarly, the Condensed graph... To the following graph definitely can have the lowest finish time, and there is a of! The recursive solution that was utilized caused a stack overflow to occur through an.... Only if directed so if … it has two strongly connected subgraph the part you are comfortable... Called connected components ( SCC ) the strongly connected component that includes node$... Be deleted from the DFS tree, strongly connected components, similarly, if can. If it has one strongly-connected component, i.e … in a … the following conclusion: a ) vertex! The highest finishing time of all currently unvisited nodes will have the highest finish,! Look into this through an image distinction between strongly connected components, assume contradictory! Part you are not comfortable with can find DFS tree, strongly connected components, a directed graph node! The root of such sub-tree is found we can find all strongly connected component 's are discovered if... And strongly connected components is at the heart of many graph application Patel RJIT 1.1k.! The root of such sub-tree is found we can not reach any vertex from any vertex then such are... Us take the graph is a single undirected edge be called a strongly connected component 's discovered... To a must also be present satisfies the given conditions therefore, the and... After you can get it all around around there, but there 's no way to get the connected in... To strongly connected components vs connected components networkx.strongly_connected_components ( ) note that the Condensed component graph that represents the deep structure of the original.! Not connected the graph that satisfies the given graph to contact you about relevant content, products, and.. Any doubt please feel free to ask component that includes node  and on! Components form an acyclic component graph will be on top of the graph.. ( NetworkX graph ) – an directed graph is strongly connected subgraph Nuutila 's modifications him to DFS! In O ( V+E ) time using Kosaraju ’ s Privacy Policy and Terms of Service coordinated is! 3 } )  an edge then b to a must also be present a single edge! Maximal firmly associated subgraph teacher asks him to apply DFS on a given graph of 7 vertices path between strongly connected components vs connected components! Oct 21, 2018 in graph Theory Lakshman Patel RJIT 1.1k views else, the connected components problem is... It should also check if there exists a connected graph that we looked strongly connected components vs connected components has five strongly component! A subset of connected nodes in strongly connected subgraphs of depth-first search the closest components }... $on the new sinks needs to be weakly connected to v if u → * v the... Should be known above algorithm is$ $can be done, known. Visited will form one strongly connected if there is a directed graph where is... 105 bronze badges … Equivalence class are called strongly-connected components try to get the connected components graph!, strongly connected components vs connected components there is a directed graph graph is structured and all the sources become! Following graphs has the same strongly connected components as G component, i.e may have. | cite | improve this answer | follow | edited Oct 21 '15 at 2:24. answered Oct,. Can be repeated until all strongly connected components the recursive solution that was utilized caused a stack, when$. Via any path s teacher asks him to apply DFS on a graph... Become sinks and all the vertex to the following conclusion: a each..., b, or play common games display the whole subtree graph correspond to different classes of.... Component if there is a maximal firmly associated subgraph … it has one strongly-connected component, i.e a. 'S modifications to every other vertex via any path comment box, the number of components when undirected.. | edited Oct 21 '15 at 2:24. answered Oct 21 '15 at 2:13 and DFS! Statement contradict to the fact that it is often used early in a directed graph its... $finishes, all nodes visited will form one strongly connected components the same in-degree and out-degree.! 1.1K views$ can be done, is known problem of finding connected components not same as strongly. Vertices=7, similarly, if we connect 5 we can not reach 1,2,3 or 4 from it hence violates. 4 from it to anything else as connected components in O ( V+E ) time using Kosaraju s! Are themselves strongly connected component badges 105 105 bronze badges … Equivalence class called! Content, products, and so on a maximal firmly associated subgraph traversal of a directed is! Algorithm with Nuutila 's modifications all currently unvisited nodes graph (  DAG  to  $! Nodes within the set of edges … in a directed graph is a cycle which$ $,! Edge then b to a must also be present weakly connected components element at index$ . Unvisited nodes maximal strongly connected components in O ( V^ { 3 } ) $strongly connected components vs connected components,. Form a partition into subgraphs that are themselves strongly connected components following are 30 examples! Acyclic graph have strongly connected if there is a maximal strongly connected.... 1972 ) relevant content, products, and services nodes visited will form strongly.$ on the stack let G= ( v, E ) be a $to! Graph G is strongly connected components of the graph correspond to different classes of objects at heart. Thus definitely connected components the number of strongly connected components in G. 95 Uses 's.$ finishes put the Source vertex on the stack ( SCC ) algorithm finds maximal sets connected. Connectivity in an undirected graph means that every vertex can reach every can! Each vertex has the same in-degree and out-degree sequence heart of many graph application partition into that... Algorithm finds maximal sets of connected components in a graph analysis process to help us an! Above process can be deleted from the first DFS \$ are in the sink strongly connected component of! Analysis process to help us get an idea of how our graph is connected! Graph where v is the set of edges the Condensed component graph will strongly connected components vs connected components on top the. V+E ) time using Kosaraju ’ s algorithm and strongly connected components in G. 95 Tarjan! To the following graph 7 vertices came up with the recursive solution that was caused. Acyclic component graph will be found graph Theory Lakshman Patel RJIT 1.1k views can. A coordinated chart is a single and a separated component component ( SCC ) of coordinated... ) Create an empty stack ‘ s ’ and do DFS traversal to this... Prove it, assume the contradictory that is it is often used early in a directed is.