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 A134876 Number of Proth primes: number of primes of the form 1 + k*2^n with k odd and k < 2^n. 3
 1, 2, 1, 3, 4, 8, 18, 23, 44, 73, 142, 277, 484, 871, 1644, 3060, 5851, 10917, 20776, 39263, 74752, 142521, 271223, 520242, 996486, 1916486, 3686628, 7103236, 13702428, 26469008, 51193351, 99099882, 192044541, 372559804, 723389144 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The ratio a(n+1)/a(n) is about 2 * n /(n+1). - Corrected by Thomas Ordowski, Oct 17 2014 Conjecture: a(n) ~ C * 2^n / n, where C = 1/(2 log 2) = 0.7213475... - Thomas Ordowski, Oct 17 2014 LINKS Eric Weisstein's World of Mathematics, Proth's Theorem EXAMPLE a(1)=1 because 3 is the only Proth prime for n=1. a(2)=2 because 5 and 13 are the only primes for n=2. a(3)=1 because 41 is the only prime for n=3. MATHEMATICA Table[cnt=0; Do[If[PrimeQ[1+k*2^n], cnt++ ], {k, 1, 2^n, 2}]; cnt, {n, 20}]] PROG (PARI) a(n) = my(s=0); forstep(k=1, 2^n-1, 2, s+=ispseudoprime(k<

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Last modified May 28 17:37 EDT 2020. Contains 334684 sequences. (Running on oeis4.)