A234142 Numbers k such that m - triangular(k) is a triangular number (A000217), where m is the least square above triangular(k).

1, 2, 3, 4, 5, 11, 12, 15, 19, 20, 22, 25, 26, 29, 32, 33, 36, 40, 43, 47, 50, 52, 54, 57, 61, 64, 68, 70, 71, 73, 75, 78, 82, 85, 89, 90, 92, 96, 99, 103, 106, 110, 113, 114, 117, 120, 121, 122, 124, 127, 131, 134, 135, 141, 148, 152, 155, 172, 173, 188, 189, 196, 213

Offset: 1

Author

Alex Ratushnyak, Dec 19 2013

Keywords

• nonn

Comments

The sequence of triangular(a(n)) begins: 1, 3, 6, 10, 15, 66, 78, 120, 190, 210, 253, 325, ...

Links

• Ivan Neretin, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000217, A000290, A234141, A234143.

Programs

Mathematica

Select[Range[215], IntegerQ@Sqrt[8 ((Floor@Sqrt[(#^2 + #)/2] + 1)^2 - (#^2 + #)/2) + 1] &] (* Ivan Neretin, May 29 2016 *)