A342660 The largest prime that divides the n-th multiply perfect number: a(n) = A006530(A007691(n)); a(1) = 1.

1, 3, 7, 5, 31, 7, 127, 7, 13, 31, 19, 31, 8191, 31, 31, 73, 73, 127, 131071, 19, 19, 89, 151, 127, 524287, 151, 127, 31, 127, 89, 89, 127, 61, 17, 3851, 1093, 151, 257, 911, 331, 2147483647, 3851, 911, 127, 257, 1093, 331, 331, 337, 337, 337, 257, 683, 3851, 8191, 1093, 257, 911, 3851, 1093, 178481, 8191, 337, 8191

Offset: 1

Author

Antti Karttunen, Mar 19 2021

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..1600

Crossrefs

Cf. A000668, A072002 (subsequences), A006530, A007691.

Cf. also A134740, A342658, A342659.

Programs

Mathematica

Map[FactorInteger[#][[-1, 1]] &, Select[Range[10^6], IntegerQ@ DivisorSigma[-1, #] &] ] (* or, using the b-file at A007691: *)
Map[FactorInteger[#][[-1, 1]] &, DeleteCases[Import["https://oeis.org/A007691/b007691.txt", "Data"], ?(Length@ # == 0 &)][[1 ;; 64, -1]] ] (* _Michael De Vlieger, Mar 19 2021 *)

Formula

a(n) = A006530(A007691(n)).