Indices k such that 13 plus the k-th triangular number is a perfect square.

Terms

a(0) =2a(1) =8a(2) =23a(3) =53a(4) =138a(5) =312a(6) =807a(7) =1821a(8) =4706a(9) =10616a(10) =27431a(11) =61877a(12) =159882a(13) =360648a(14) =931863a(15) =2102013a(16) =5431298a(17) =12251432a(18) =31655927a(19) =71406581a(20) =184504266a(21) =416188056a(22) =1075369671a(23) =2425721757a(24) =6267713762a(25) =14138142488a(26) =36530912903a(27) =82403133173a(28) =212917763658a(29) =480280656552