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A098920
Least k such that k*M#(n) + 1 is prime where M#(n) is the product of the first n Mersenne primes = Product_{j=1..n} A000668(j).
0
2, 2, 2, 6, 30, 10, 4, 26, 12, 10, 12, 436, 644, 424, 2164, 862, 408, 9558, 5226, 12350
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
Sequence in context: A321741 A101416 A371919 * A270557 A129365 A125838
KEYWORD
hard,more,nonn
AUTHOR
Pierre CAMI, Oct 17 2004
EXTENSIONS
a(15)-a(20) from Donovan Johnson, Mar 23 2008
STATUS
approved