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A112729
Numbers of the form 2^(n+2)*3*(7*2^n-1) where 7*2^n-1 is prime.
2
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.
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
KEYWORD
easy,nonn
AUTHOR
Farideh Firoozbakht, Dec 01 2005
STATUS
approved