Althoughthegreyforecastingmodelhasbeensuccessfullyadoptedinvariousfieldsanddemonstratedpromisingresults,theliteraturesshowitsperformancecouldbefurtherimproved,suchasfortheDGM(1,1)model,basedonaconcavesequence,themodelingerrorwillbelarger.Inthispaper,firstlythedefinitionofsequenceconvexityisgivenout,anditisprovedthattheoutputsequenceofDGM(1,1)modelisaconvexsequence.Next,theresidualchangelawofDGM(1,1)modelbasedontheconcavesequenceisdiscussed,andthenon-equidistanceDGM(1,1)modelisproposed.Finally,byintroducingthesymmetrytransformation,aconcavesequenceistransformedintoaconvexsequence,calledthesymmetricsequenceoftheconcavesequence,andthenconstructthenon-equidistanceDGM(1,1)modelbasedontheconvexsequence.Theexampleresultsshowthatthenovelmethodismoreaccuratethanthedirectmodelingforaconcavesequence.
