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A135242 Numbers n such that sigma(n)=2*(n-reversal(n)). 1

%I

%S 10,51,430,591,922,5991,8350,20210,28810,59211,59991,6673581,8179030,

%T 20158910,203887910,833549650,2263048910,2814442310,7823347543,

%U 78551146723,81896108650,281444442310,770920338133,832428872110,7670112536751

%N Numbers n such that sigma(n)=2*(n-reversal(n)).

%C If n=6*10^m-9 is semiprime then n is in the sequence (the proof is easy).

%C a(26) > 10^13. - _Giovanni Resta_, Jul 13 2015

%e sigma(51)=72=2*(51-15)=2*(51-reversal(51)), so 51 is in the sequence.

%t Do[If[2(n-FromDigits@Reverse@IntegerDigits@n)==DivisorSigma[1,n], Print[n]],{n,70000000}]

%Y Cf. A135243.

%K base,more,nonn

%O 1,1

%A _Farideh Firoozbakht_, Jan 04 2008

%E a(15)-a(21) from _Donovan Johnson_, Dec 21 2008

%E a(22)-a(25) from _Giovanni Resta_, Jul 13 2015

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Last modified August 3 06:22 EDT 2021. Contains 346435 sequences. (Running on oeis4.)