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A211395
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Number of Sophie Germain primes between 2^n and 2^(n+1).
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2
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1, 1, 1, 1, 2, 2, 3, 7, 8, 13, 23, 41, 67, 111, 193, 360, 630, 1091, 1938, 3558, 6448, 11876, 21649, 40151, 73658, 135711, 251786, 468678, 875247, 1634069, 3060794, 5746245, 10806204, 20356921, 38433398, 72656139, 137562095
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OFFSET
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0,5
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COMMENTS
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To be precise, the number of Sophie Germain primes p, 2^n < p <= 2^(n+1). Since 2 is a Sophie Germain prime, this precise definition is important only for determining a(0) and a(1). The alternative definition (with 2^n <= p < 2^(n+1)) would give the sequence 0, 2, 1, 1, 2, 2, 3, 7, 8, 13, 23, 41, 67, 111, 193, ...
The Sophie Germain primes p are in A005384. The corresponding primes s = 2p + 1 are called safe primes, and are in A005385. The number of safe primes between 2^(n+1) and 2^(n+2) is given by the sequence in the previous paragraph.
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LINKS
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FORMULA
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MATHEMATICA
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nmax = 36; rtable = Table[0, {nmax}];
Do[r = 0;
Do[If[And[PrimeQ[i], PrimeQ[2 i + 1]], r++], {i, 1 + 2^n,
2^(n + 1)}]; Print[n, " ", r];
rtable[[n + 1]] = r, {n, 0, nmax - 1}];
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PROG
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(PARI) a211395(n) = {local(r, i); r=0; for(i=2^n+1, 2^(n+1), if(isprime(i)&&isprime(2*i+1), r=r+1)); r} \\ Michael B. Porter, Feb 08 2013
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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