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Palindromes for which both the numerator (A017665) and the denominator (A017666) of sigma(n)/n are palindromes, where sigma is the sum of divisors (A000203).
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%I #21 Sep 29 2014 12:21:12

%S 1,2,3,4,5,6,7,333,17571,40004,93939,569965,1787871,2316132,541626145,

%T 17575757571,5806270726085,7359770779537,520524424425025,

%U 17275787578757271,17878787578787871

%N Palindromes for which both the numerator (A017665) and the denominator (A017666) of sigma(n)/n are palindromes, where sigma is the sum of divisors (A000203).

%C Compare with A028986 (Palindromes whose sum of divisors is palindromic).

%C These terms of A028986 also belong here: 1, 2, 3, 4, 5, 7, 333, 17571, 1787871, 541626145, 17575757571, 5806270726085, 7359770779537.

%C a(22) > 10^18. - _Hiroaki Yamanouchi_, Sep 27 2014

%o (PARI) reverse(expr)=my(v=Vec(Str(expr)),n=length(v));eval(concat(vector(n,i,v[n-i+1])));

%o isok(n) = (rn = reverse(n)) && (rn == n) && (ab = sigma(n)/n) && (abr = sigma(rn)/rn) && (numerator(abr) == reverse(numerator(ab))) && (denominator(abr) == reverse(denominator(ab)));

%Y Cf. A000203, A017665, A017666, A028986.

%K nonn,base,more

%O 1,2

%A _Michel Marcus_, Apr 06 2014

%E a(16)-a(21) from _Hiroaki Yamanouchi_, Sep 27 2014