A112729 Numbers of the form 2^(n+2)*3*(7*2^n-1) where 7*2^n-1 is prime.

312, 85632, 22013952, 1443107438592, 369435881766912, 24211351590301335552, 103986963299971520879061368832

Offset

1,1

Comments

This sequence is a subsequence of A011251 and A011774, namely if m is in the sequence then phi(m)+sigma(m)=3*m (see Comments line of A011251).

Number of digits of all the 20 known terms of this sequence are respectively 3, 5, 8, 13, 15, 20, 30, 109, 11069, 13566, 14787, 15722, 20988, 25263, 40594, 42272, 101802, 104453, 107155 and 219110.

Links

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

Example

103986963299971520879061368832 is in the sequence because 103986963299971520879061368832=2^(45+2)*3*(7*2^45-1) and 7*2^45-1 is prime.

Mathematica

Do[If[PrimeQ[7*2^n-1], Print[3*2^(n+2)*(7*2^n-1)]], {n, 177}]

Crossrefs

Cf. A001771. A011251, A011774.

Sequence in context: A015704 A156406 A283492 * A308837 A239818 A290181

Adjacent sequences: A112726 A112727 A112728 * A112730 A112731 A112732

Keyword

easy,nonn

Author

Farideh Firoozbakht, Dec 01 2005

Status

approved