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Number of k such that (2^k-1)*2^n+1 is prime with 0 < k < n+1.

4

`%I #17 Sep 21 2019 08:28:06
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`%S 1,2,0,3,1,2,0,3,2,2,0,3,1,4,0,3,2,5,2,4,2,1,1,2,3,2,1,2,2,3,0,4,2,4,
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`%T 1,2,4,7,0,4,3,3,0,4,5,1,0,3,2,2,1,1,2,2,0,2,3,2,1,4,0,5,0,2,5,4,0,7,
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`%U 4,2,2,7,0,3,1,2,0,5,0,2,3,1,0,4,4,0,0,1,0,8
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`%N Number of k such that (2^k-1)*2^n+1 is prime with 0 < k < n+1.
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`%C As n increases sum a(n) from n=1 to n / n tends to 2.66.
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`%H Pierre CAMI, <a href="/A197117/b197117.txt">Table of n, a(n) for n = 1..3000</a>
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`%o (PARI) a(n) = sum(k=1, n, isprime((2^k-1)*2^n+1)); \\ _Michel Marcus_, Sep 21 2019
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`%Y Cf. A197116, A197118, A197119.
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`%K nonn
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`%O 1,2
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`%A _Pierre CAMI_, Oct 11 2011
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