cp's OEIS Frontend

A203615 Reversal of sigma(n) equals the sum of the reversals of the divisors of n.

%I A203615 #18 Sep 01 2018 11:01:57
%S A203615 1,2,3,4,5,7,21,938,17797,44045,87001,454085,2217425,8156450,8475789,
%T A203615 3293216050,11130063842,44662814795,77084972662
%N A203615 Reversal of sigma(n) equals the sum of the reversals of the divisors of n.
%C A203615 a(20) > 2.34*10^12. - _Giovanni Resta_, Aug 30 2018
%e A203615 n=17797. Divisors: 1, 13, 37, 481, 1369, 17797.
%e A203615 Sum of the reversals of the divisors: 1+31+73+184+9631+79771=89691.
%e A203615 Sigma(17797)=19698 and its reversal is 89691.
%e A203615 n=454085. Divisors: 1, 5, 197, 461, 985, 2305, 90817, 454085.
%e A203615 Sum of the reversals of the divisors: 1+5+791+164+589+5032+71809+580454=658845.
%e A203615 Sigma(454085)=548856 and its reversal is 658845.
%p A203615 with(numtheory);
%p A203615 Rev:=proc(n)
%p A203615 local a, i, k;
%p A203615   i:=convert(n,base,10); a:=0;
%p A203615   for k from 1 to nops(i) do a:=a*10+i[k]; od;
%p A203615   a;
%p A203615 end:
%p A203615 P:=proc(s)
%p A203615 local a, b, c, j, pfs;
%p A203615 for j from 1 to s do
%p A203615   b:=divisors(j); a:=0;
%p A203615   for c from 1 to nops(b) do a:=a+Rev(b[c]); od;
%p A203615   if Rev(sigma(j))=a then print(j); fi;
%p A203615 od;
%p A203615 end:
%p A203615 P(10000000);
%t A203615 Select[Range[33*10^8],Total[IntegerReverse/@Divisors[#]] == IntegerReverse[ DivisorSigma[ 1,#]]&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Apr 09 2018 *)
%Y A203615 Cf. A069942, A195144, A203616.
%K A203615 nonn,base,more
%O A203615 1,2
%A A203615 _Paolo P. Lava_, Jan 20 2012
%E A203615 a(13)-a(16) from _Donovan Johnson_, Jan 29 2012
%E A203615 a(17)-a(19) from _Giovanni Resta_, Aug 30 2018