A230020 - OEIS

%I #10 May 04 2021 14:39:16

%S 9599,31874,93689,932639,1367991,13451831,309173753,99148830299

%N Composite numbers n such that sigma(n) - reversal(n) = 1.

%C It is obvious that palindromic primes (palprimes) are all prime solutions of the equation sigma(x)-reversal(x) = 1. The next term is greater than 73*10^7.

%C a(9) > 10^12. - _Giovanni Resta_, Dec 14 2013

%e sigma(9599) = sigma(29*331) = 30*332 = 9960 = reverse(9599)+1.

%t Do[a=FromDigits[Reverse[IntegerDigits[n]]]; If[a>n && DivisorSigma[1,n] -a == 1, Print[n]], {n,730000000}]

%Y Cf. A000203, A002385, A004086, A130000.

%K nonn,base,more

%O 1,1

%A _Farideh Firoozbakht_, Nov 26 2013

%E a(8) from _Giovanni Resta_, Dec 14 2013