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A331539 a(n) gives the number of primes of form (2*n+1)*2^m + 1 where m satisfies 2^m <= 2*n+1. 1
1, 1, 1, 1, 3, 2, 1, 2, 1, 0, 2, 1, 2, 2, 2, 0, 1, 2, 2, 4, 1, 1, 1, 0, 1, 2, 2, 1, 2, 1, 1, 3, 3, 2, 2, 2, 2, 4, 1, 1, 3, 2, 2, 2, 1, 0, 3, 3, 2, 4, 1, 0, 3, 1, 1, 2, 2, 1, 3, 2, 0, 1, 2, 1, 2, 2, 2, 4, 1, 1, 4, 0, 1, 0, 2, 1, 2, 2, 0, 2, 2, 3, 5, 1, 1, 0, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,5
COMMENTS
For each index n, let k = 2*n+1. Then a(n) gives the number of primes of form k*2^m + 1 that are NOT considered Proth primes (A080076) because their m are too small.
In the edge case n=0, so k=1, we count 1*2^0 + 1 = 2 as a non-Proth prime.
LINKS
EXAMPLE
For n=10, we consider 21*2^m + 1, where m runs from 0 to 4 (the next value m=5 would make 2^m exceed 21). The number of cases where 21*2^m + 1 is prime, is 2, namely m=1 (prime 43) and m=4 (prime 337). So 2 primes means a(10)=2. Compare with the start of A032360, all k=21 primes.
MATHEMATICA
a[n_] := Sum[Boole @ PrimeQ[(2n+1)*2^m + 1], {m, 0, Log2[2n+1]}]; Array[a, 100, 0] (* Amiram Eldar, Jan 20 2020 *)
PROG
(PARI) a(n) = my(k=2*n+1); sum(m=0, logint(k, 2), ispseudoprime(k<<m+1))
CROSSREFS
Sequence in context: A136170 A245188 A137241 * A306287 A016457 A181715
KEYWORD
nonn
AUTHOR
Jeppe Stig Nielsen, Jan 19 2020
STATUS
approved

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Last modified June 12 15:40 EDT 2024. Contains 373331 sequences. (Running on oeis4.)