A230013 Numbers n such that phi(n) + sigma(n) = (3/2)*reversal(n).

2993, 29993, 2999993, 230880803, 299999993, 2999999993, 2999999999993

Offset: 1

Author

Farideh Firoozbakht, Nov 30 2013

Keywords

• nonn
• base
• more

Comments

All semiprimes of the form 3*10^m-7 are in the sequence.

Next term is greater than 10^8.

Crossrefs

Cf. A000010, A000203, A004086, A136543.

Programs

Mathematica

r[n_]:=FromDigits[Reverse[IntegerDigits[n]]];Do[If[c=r[n]; c>(4/3)*n && DivisorSigma[1,n] + EulerPhi[n] == (3/2)c, Print[n]], {n,100000000}]

Extensions

a(4)-a(7) from Giovanni Resta, Feb 06 2014