A114928 Numbers n such that sigma(n)=4*reversal(n).

42, 402, 492, 4000002, 57906504, 400000002, 4000000002, 6279090751, 62698513951, 400000000002

Offset: 1

Author

Farideh Firoozbakht, Jan 28 2006

Keywords

• base
• more
• nonn

Comments

If p=(2*10^n+1)/3 is prime then m=6*p is in the sequence because sigma(m)=sigma(6*p)=12*(2*10^n+4)/3=4*(2*10^n+4)=4* reversal(4*10^n+2)=4*reversal(6*(2*10^n+1)/3)=4*reversal(6*p) =4*reversal(m). Next term is greater than 5*10^8.

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

Examples

			492 is in the sequence because sigma(492)=sigma(4*3*41)=7*4*42
=4*294=4*reversal(492).
		

Crossrefs

Cf. A069216, A105324, A114927.

Programs

Mathematica

Do[If[DivisorSigma[1, n]==4*FromDigits[Reverse[IntegerDigits[n]]], Print[n]], {n, 500000000}]

Extensions

a(7)-a(9) from Donovan Johnson, Dec 21 2008

a(10) from Giovanni Resta, Oct 28 2012