A235710 - OEIS

%I #16 Mar 21 2014 20:07:06

%S 14,124,194,1324,1994,13324,133324,1130324,1333324,13333324,62496048,

%T 133333324,92782317392

%N Composite numbers n such that sum of the proper divisors of n is a power of 10.

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

%C If n is prime then sum of the proper divisors of n is 10^0.

%C If m is a natural number and p=10^m-3 is prime then 2*p is in the sequence.

%C If m is a natural number and p=(10^m-7)/3 is prime then 4*p is in the sequence.

%C a(14) > 3*10^12. - _Giovanni Resta_, Mar 21 2014

%e sigma(14)-14 = 1+2+7 = 10, sigma(124)-124 = 1+2+4+31+62 = 100.

%t r[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Do[If[!PrimeQ[n]&& r[DivisorSigma[1,n]-n]==1, Print[n]],{n, 200000000}]

%Y Cf. A000203, A004086.

%K nonn,base,more

%O 1,1

%A _Farideh Firoozbakht_, Mar 17 2014

%E a(13) from _Giovanni Resta_, Mar 21 2014