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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. 1

%I

%S 24,31,42,48,56,60,72,84,90,96,120,124,126,144,156,168,180,186,192,

%T 210,216,224,228,234,240,248,252,270,280,288,294,312,320,324,336,342,

%U 360,372,378,384,392,399,403,408,416,420,432,434,448,450,456,468,480,504

%N 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.

%e Number 24 is in sequence because 24 = sigma(x) for composite numbers x = 14 and 15.

%t 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 *)

%Y Cf. A206447 (composite numbers n such that sigma(n) = sigma(d) has solution for any other composite number d), A000203.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Feb 07 2012

%E Corrected by T. D. Noe, Feb 13 2012

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Last modified September 26 23:23 EDT 2021. Contains 347673 sequences. (Running on oeis4.)