A206448 Values of sigma(n) of composite numbers n such that sigma(n) = sigma(d) has solution for any other composite number d in increasing order.
24, 31, 42, 48, 56, 60, 72, 84, 90, 96, 120, 124, 126, 144, 156, 168, 180, 186, 192, 210, 216, 224, 228, 234, 240, 248, 252, 270, 280, 288, 294, 312, 320, 324, 336, 342, 360, 372, 378, 384, 392, 399, 403, 408, 416, 420, 432, 434, 448, 450, 456, 468, 480, 504
Offset: 1
Keywords
Examples
Number 24 is in sequence because 24 = sigma(x) for composite numbers x = 14 and 15.
Crossrefs
Programs
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Mathematica
nn = 600; t = Table[If[PrimeQ[n], -1, DivisorSigma[1, n]], {n, nn}]; t2 = Rest[Select[Sort[Tally[t]], #[[2]] > 1 &]]; Select[Transpose[t2][[1]], # <= nn &] (* T. D. Noe, Feb 13 2012 *)
Extensions
Corrected by T. D. Noe, Feb 13 2012