What is a Knowledge Graph?

A knowledge graph is a directed labeled graph in which the labels have well-defined meanings. A directed labeled graph consists of nodes, edges, and labels.   CS520 KnowledgeGraphs WhatshouldAIKnow?   WhatisaKnowledgeGraph? 1.Introduction Knowledgegraphshaveemergedasacompellingabstractionfor organizingworld'sstructuredknowledgeovertheinternet,andaway tointegrateinformationextractedfrommultipledata sources.Knowledgegraphshavealsostartedtoplayacentralrolein machinelearningasamethodtoincorporateworldknowledge,asa targetknowledgerepresentationforextractedknowledge,andfor explainingwhatislearned. Ourgoalhereistoexplainthebasicterminology,conceptsand usageofknowledgegraphsinasimpletounderstandmanner.Wedonot intendtogivehereanexhaustivesurveyofthepastandcurrentwork onthetopicofknowledgegraphs. Wewillbeginbydefiningknowledgegraphs,someapplicationsthat havecontributedtotherecentsurgeinthepopularityofknowledge graphs,andthenuseofknowledgegraphsinmachinelearning.We willconcludethischapterbysummarizingwhatisnewanddifferent abouttherecentuseofknowledgegraphs. 2.KnowledgeGraphDefinition Aknowledgegraphisadirectedlabeledgraphinwhichthelabels havewell-definedmeanings.Adirectedlabeledgraphconsistsof nodes,edges,andlabels.Anythingcanactasanode,forexample, people,company,computer,etc.Anedgeconnectsapairofnodesand capturestherelationshipofinterestbetweenthem,forexample, friendshiprelationshipbetweentwopeople,customerrelationship betweenacompanyandperson,oranetworkconnectionbetweentwo computers.Thelabelscapturethemeaningoftherelationship,for example,thefriendshiprelationshipbetweentwopeople. Moreformally,givenasetofnodesN,andasetof labelsL,aknowledgegraphisasubsetofthecross productN×L×N.Eachmemberofthissetis referredtoasatriple,andcanbevisualizedasshownbelow. Thedirectedgraphrepresentationisusedinavarietyofways dependingontheneedsofanapplication.Adirectedgraphssuchas theoneinwhichthenodesarepeople,andtheedgescapture friendshiprelationshipisalsoknownasadatagraph.Adirected graphinwhichthenodesareclassesofobjects(e.g.,Book, Textbook,etc.),andtheedgescapturethesubclass relationship,isalsoknownasataxonomy.Insomedata models,Aisreferredtoassubject,Bis referredtoaspredicate,andCisreferredtoas object. Manyinterestingcomputationsovergraphscanbereducedto navigation.Forexample,inafriendshipknowledgegraph, tocalculatethefriendsofafriendsofapersonA,wecan navigatetheknowledgegraphfromAtoallnodesB connectedtoitbyarelationlabeledasfriend,andthen recursivelytoallnodesCconnectedbythefriendrelationto eachB. ApathinagraphGisaseriesofnodes (v1,v2,...,vn)whereforanyi ∈Nwith1≤i