A249671 - OEIS

%I #16 Nov 10 2014 17:45:17

%S 1,364,86800,90768,149856,632400,828816,46462800,61987600,148468320,

%T 153972000,185684160,187465824,189300384,191659104,193808160,

%U 196219296,202460352,204118464,215399520,220116960,221931360,222778080,223382880,228584160,338116680

%N Integers n such that A249670(n) = n.

%C That is, integers n such that A017665(n)*A017666(n) = n.

%e sigma(364)/364 = 28/13 and 28*13 = 364, so 364 is in the sequence.

%t lista[n_Integer] := Select[Range[n], # == Numerator[DivisorSigma[-1, #]] * Denominator[DivisorSigma[-1, #]] &];

%t lista[10^6] (* _Michael De Vlieger_, Nov 10 2014 *)

%o (PARI) isok(n) = my(ab = sigma(n)/n); numerator(ab)*denominator(ab) == n; \\ _Michel Marcus_, Nov 04 2014

%Y Cf. A017665, A017666, A249670.

%K nonn

%O 1,2

%A _Michel Marcus_, Nov 03 2014