A136543 Numbers n such that phi(n)+sigma(n)=4*reversal(n).

793, 79993, 2152311, 79999993, 7999999993, 799999999993

Offset: 1

Author

Farideh Firoozbakht, Jan 20 2008

Keywords

• base
• more
• nonn

Comments

All semiprimes of the form 8*10^m-7 are in the sequence - the proof is easy. Next term is greater than 10^8.

a(7) > 10^12. - Giovanni Resta, Nov 03 2012

A100412(n) is in the sequence for n = 2, 4, 7, 9, 11, 16, 18, 23, 31, 32, 40,... - M. F. Hasler, Nov 03 2012

Examples

			phi(2152311)+sigma(2152311)=1217664+3312384=4*1132512=4*reversal(2152311), so 2152311 is in the sequence.
		

Programs

Mathematica

Do[If[4*FromDigits@Reverse@IntegerDigits@n==EulerPhi@n+ DivisorSigma[1,n],Print[n]],{n,100000000}]

Extensions

a(5)-a(6) from Giovanni Resta, Nov 03 2012