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A238735 Number of prime pairs {2^n + (2k + 1), (2k + 1)*2^n + 1}, k < n. 1
1, 2, 1, 2, 0, 3, 2, 2, 0, 1, 0, 2, 0, 0, 0, 3, 0, 1, 0, 3, 0, 1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If k = 0, then the two numbers in the "prime pair" are actually the same number, 2^n + 1 (which is either 2 or a Fermat prime; see A019434, A092506).

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..631

EXAMPLE

a(1) = 1 because 2^1+(2*0+1)=3 and (2*0+1)*2^1+1=3 is prime pair for k=0,

a(2) = 2 because 2^2+(2*0+1)=5 and (2*0+1)*2^2+1=5 is prime pair for k=0, 2^2+(2*1+1)=7 and (2*1+1)*2^2+1=13 is prime pair for k=1,

a(3) = 1 because 2^3+(2*2+1)=13 and (2*2+1)*2^3+1=41 is prime pair 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, 100] (* Giovanni Resta, Mar 04 2014 *)

PROG

(PARI) a(n)=sum(k=0, n-1, isprime(2^n+2*k+1)&&isprime((2*k+1)<<n+1)) \\ Charles R Greathouse IV, Mar 06 2014

CROSSREFS

Cf. A019434, A238554.

Sequence in context: A271484 A199920 A177995 * A258120 A147786 A275019

Adjacent sequences:  A238732 A238733 A238734 * A238736 A238737 A238738

KEYWORD

nonn

AUTHOR

Ilya Lopatin and Juri-Stepan Gerasimov, Mar 04 2014

EXTENSIONS

a(47)-a(87) from Giovanni Resta, Mar 04 2014

STATUS

approved

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Last modified November 15 22:20 EST 2018. Contains 317252 sequences. (Running on oeis4.)