OFFSET
1,1
EXAMPLE
2*(2^2-1)*(2^3-1)*(2^5-1) + 1 = 1303 prime so a(3)=2.
MATHEMATICA
With[{s = FoldList[Times, Array[2^MersennePrimeExponent@ # - 1 &, 16]]}, Array[Block[{k = 2}, While[! PrimeQ[k s[[#]] + 1], k++]; k] &, Length@ s]] (* Michael De Vlieger, Dec 27 2019 *)
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Pierre CAMI, Oct 17 2004
EXTENSIONS
a(15)-a(20) from Donovan Johnson, Mar 23 2008
STATUS
approved