OFFSET
1,2
COMMENTS
The indices of these triangular numbers are 1, 2, 11, 20, 23, 47, 68, 71, 92, 107, 119, 124, 125, 153, 161, 191, 212, 236, 240, ...
The indices of the square psi values are 1, 2, 12, 24, 24, 48, 72, 72, 96, 108, 144, 120, 120, 144, 144, 192, 216, 240, 264, ...
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
66 is in the sequence since 66 = 11*12/2 is triangular, and psi(66) = 144 = 12^2 is square.
MATHEMATICA
psi[n_] := If[n<1, 0, n*Sum[MoebiusMu[d]^2/d, {d, Divisors @ n}]];
Select[Accumulate[Range[1000]], IntegerQ[Sqrt[psi[#]]]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Sep 08 2017
STATUS
approved