A308494 Refactorable numbers k such that the number of divisors of k-1 also divides k.

8, 12, 18, 24, 36, 40, 56, 60, 72, 80, 84, 88, 96, 104, 108, 128, 132, 136, 152, 156, 180, 184, 204, 228, 232, 240, 248, 252, 276, 288, 296, 328, 344, 348, 360, 372, 376, 384, 396, 444, 448, 450, 468, 472, 480, 488, 492, 504, 516, 536, 560, 564, 584, 600

Offset

1,1

Links

Table of n, a(n) for n=1..54.

Eric Weisstein's World of Mathematics, Refactorable Number

Mathematica

Flatten[Table[If[Sum[(1 - Ceiling[n/DivisorSigma[0, n]] + Floor[n/DivisorSigma[0, n]]) (1 - Ceiling[n/DivisorSigma[0, n - 1]] + Floor[n/DivisorSigma[0, n - 1]]) , {i, Floor[(n - 1)/2]}] > 0, n, {}], {n, 0, 600}]]

Crossrefs

Cf. A033950.

Sequence in context: A085103 A280968 A157940 * A087696 A293529 A028393

Adjacent sequences: A308491 A308492 A308493 * A308495 A308496 A308497

Keyword

nonn

Author

Wesley Ivan Hurt, Jul 27 2019

Status

approved