Numbers k such that 2 * A000217(k) + 1 is triangular.

Terms

a(0) =0a(1) =1a(2) =4a(3) =9a(4) =26a(5) =55a(6) =154a(7) =323a(8) =900a(9) =1885a(10) =5248a(11) =10989a(12) =30590a(13) =64051a(14) =178294a(15) =373319a(16) =1039176a(17) =2175865a(18) =6056764a(19) =12681873a(20) =35301410a(21) =73915375a(22) =205751698a(23) =430810379a(24) =1199208780a(25) =2510946901a(26) =6989500984a(27) =14634871029a(28) =40737797126a(29) =85298279275