A115749 Numbers n such that sigma(n)=8*reversal(n).

861, 951, 2070, 8241, 900051, 8864151, 9000051, 82000041, 8200000041, 82000000041

Offset: 1

Author

Farideh Firoozbakht, Feb 12 2006

Keywords

• base
• more
• nonn

Comments

If p=3*10^n+17 is prime then 3*p is in the sequence because sigma(3*p)=4*(3*10^n+18)=12*10^n+72=8*(15*10^(n-1)+9)=8* reversal(9*10^n+51)=8*reversal(3*p). Also if p=(2*10^n+1)/3 is prime then 123*p is in the sequence (the proof is easy). Next term is greater than 13*10^7.

a(11) > 10^12. - Giovanni Resta, Oct 28 2012

Examples

			82000041 is in the sequence because sigma(82000041)
=112000224=8*14000028=8*reversal(82000041).
		

Crossrefs

Cf. A069216, A105324, A114928, A115747, A115748.

Programs

Mathematica

Do[If[DivisorSigma[1,n]==8*FromDigits[Reverse[IntegerDigits[n]]],Print[n]],{n,130000000}]

Extensions

a(9)-a(10) from Donovan Johnson, Dec 21 2008