Indices k such that 4 plus the k-th triangular number is a perfect square.

Terms

a(0) =0a(1) =6a(2) =9a(3) =39a(4) =56a(5) =230a(6) =329a(7) =1343a(8) =1920a(9) =7830a(10) =11193a(11) =45639a(12) =65240a(13) =266006a(14) =380249a(15) =1550399a(16) =2216256a(17) =9036390a(18) =12917289a(19) =52667943a(20) =75287480a(21) =306971270a(22) =438807593a(23) =1789159679a(24) =2557558080a(25) =10427986806a(26) =14906540889a(27) =60778761159a(28) =86881687256