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 A237422 Number of prime pairs {2^n - (2k + 1), (2k + 1)*2^n - 1}, k < n. 2
 0, 1, 2, 2, 1, 1, 1, 1, 0, 2, 0, 2, 2, 1, 0, 1, 1, 1, 1, 1, 2, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 1, 3, 0, 0, 0, 1, 2, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 2, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS If k = 0, then the two numbers in the "prime pair" are actually the same number, 2^n - 1 (a Mersenne prime; see A000668). LINKS EXAMPLE a(2) = 1 because 2^2-(2*0+1)=3 and (2*0+1)*2^2-1=3 for k=0; a(3) = 2 because 2^3-(2*0+1)=7 and (2*0+1)*2^3-1=7 for k=0, 2^3-(2*1+1)=5 and (2*1+1)*2^3-1=23 for k=1; a(4) = 2 because 2^4-(2*1+1)=13 and (2*1+1)*2^4-1)=47 for k=1, 2^4-(2*2+1)=11 and (2*2+1)*2^4-1=59 for k=2. MATHEMATICA a[n_] := Length@Select[Range[0, n-1], PrimeQ[2^n - (2*#+1)] && PrimeQ[(2*#+1) * 2^n-1] &]; Array[a, 90] (* Giovanni Resta, Mar 04 2014 *) CROSSREFS Cf, A000043, A238694. Sequence in context: A054535 A054534 A085769 * A102552 A131341 A124034 Adjacent sequences:  A237419 A237420 A237421 * A237423 A237424 A237425 KEYWORD nonn AUTHOR Ilya Lopatin and Juri-Stepan Gerasimov, Mar 04 2014 EXTENSIONS a(6), a(42), a(48)-a(87) from Giovanni Resta, Mar 04 2014 STATUS approved

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Last modified January 16 06:48 EST 2021. Contains 340204 sequences. (Running on oeis4.)