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%I #5 Mar 30 2012 17:37:43
%S 312,85632,22013952,1443107438592,369435881766912,
%T 24211351590301335552,103986963299971520879061368832
%N Numbers of the form 2^(n+2)*3*(7*2^n-1) where 7*2^n-1 is prime.
%C 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).
%C 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.
%e 103986963299971520879061368832 is in the sequence because 103986963299971520879061368832=2^(45+2)*3*(7*2^45-1) and 7*2^45-1 is prime.
%t Do[If[PrimeQ[7*2^n-1], Print[3*2^(n+2)*(7*2^n-1)]], {n, 177}]
%Y Cf. A001771. A011251, A011774.
%K easy,nonn
%O 1,1
%A _Farideh Firoozbakht_, Dec 01 2005