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Numbers n such that numerator(sigma(n)/n) = reverse(denominator(sigma(n)/n)).
1

%I #31 Jul 15 2015 14:04:20

%S 1,69,879,1456,67984,100464,185562,899979,1936935,3043383,5205564,

%T 6638505,14166720,15332352,24111360,31231431,59750964,212464896,

%U 354078153,445368330,1058445648,2317488316,2761626465,2869182400,3097104192,3296170752,4179222528,4675532364

%N Numbers n such that numerator(sigma(n)/n) = reverse(denominator(sigma(n)/n)).

%H Antoine Mathys, <a href="/A259398/b259398.txt">Table of n, a(n) for n = 1..35</a>

%e For n=1, sigma(1)/1 = 1/1.

%e For n=69, sigma(69)/69 = 32/23.

%t fQ[n_]:=Numerator[DivisorSigma[1,n]/n]==FromDigits[Reverse[ IntegerDigits[Denominator[DivisorSigma[1,n]/n]]]];

%t Select[Range[10^6],fQ[#]&] (* _Ivan N. Ianakiev_, Jun 30 2015 *)

%o (PARI) isok(n) = my(ab= sigma(n)/n); digits(numerator(ab)) == Vecrev(digits(denominator(ab)));

%Y Cf. A004086, A017665, A017666.

%K nonn,base,hard

%O 1,2

%A _Michel Marcus_, Jun 26 2015

%E a(21) from _Ivan N. Ianakiev_, Jun 30 2015

%E a(22)-a(28) from _Antoine Mathys_, Jul 01 2015