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