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

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

Offset

1,1

Comments

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

Next term is greater than 10^8.

Links

Table of n, a(n) for n=1..7.

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}]

Crossrefs

Cf. A000010, A000203, A004086, A136543.

Sequence in context: A253962 A253955 A253758 * A158963 A178326 A063055

Adjacent sequences: A230010 A230011 A230012 * A230014 A230015 A230016

Keyword

nonn,base,more

Author

Farideh Firoozbakht, Nov 30 2013

Extensions

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

Status

approved