A387920 - OEIS

A387920

Numbers k such that s(k) = s(k+2), where s(k) is the sum of odd divisors of k (A000593).

2, 33, 2913, 4187, 7169, 9359, 12565, 15085, 21461, 24881, 25019, 26609, 46863, 49225, 51835, 53963, 54178, 59987, 77057, 81055, 101101, 127585, 143369, 149149, 150419, 170561, 205727, 215069, 220817, 230578, 280787, 292315, 319955, 334983, 336505, 360325, 360685

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OFFSET

1,1

COMMENTS

The even terms, 2, 54178, 230578, ..., are twice the terms of A164522.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..1000

MATHEMATICA

s[n_] := s[n] = DivisorSigma[1, n/2^IntegerExponent[n, 2]]; Select[Range[10^5], s[#] == s[#+2] &]

PROG

(PARI) s(n) = sigma(n >> valuation(n, 2));

list(kmax) = {my(s1 = s(1), s2 = s(2), s3); for(k=3, kmax, s3 = s(k); if(s3 == s1, print1(k-2, ", ")); s1 = s2; s2 = s3); }