A377758 a(n) is the least i such that 2n-prime(i) is also a prime, where prime(i) is the i-th prime.

1, 2, 2, 2, 3, 2, 2, 3, 2, 2, 3, 2, 3, 4, 2, 2, 3, 4, 2, 3, 2, 2, 3, 2, 3, 4, 2, 3, 4, 2, 2, 3, 4, 2, 3, 2, 2, 3, 4, 2, 3, 2, 3, 4, 2, 3, 4, 8, 2, 3, 2, 2, 3, 2, 2, 3, 2, 3, 4, 6, 5, 6, 8, 2, 3, 2, 3, 4, 2, 2, 3, 4, 5, 5, 2, 2, 3, 4, 2, 3, 4, 2, 3, 2, 3

Offset

2,2

Links

Robert Israel, Table of n, a(n) for n = 2..10000

Formula

a(n) = pi(A020481(n)).

Maple

f:= proc(n) local i, p;
  p:= 1:
  for i from 1 do
    p:= nextprime(p);
    if isprime(2*n-p) then return i fi
  od
end proc:
map(f, [$2..100]); # Robert Israel, Nov 19 2024

Mathematica

Table[Module[{i = 1}, While[i <= PrimePi[n] && ! PrimeQ[n - Prime[i]], i++]; If[i <= PrimePi[n], i, None]], {n, 4, 1000, 2}]

Prog

(PARI) a(n) = my(i=1); while (!isprime(2*n-prime(i)), i++); i; \\ Michel Marcus, Nov 06 2024
(Python)
from sympy import primerange, isprime
def A377758(n): return next(i for i, p in enumerate(primerange(2*n), 1) if isprime((n<<1)-p)) # Chai Wah Wu, Nov 19 2024

Crossrefs

Cf. A000040, A000720, A020481, A377972.

Sequence in context: A173883 A022922 A239141 * A195352 A103507 A219252

Adjacent sequences: A377753 A377754 A377755 * A377759 A377760 A377761

Keyword

nonn,new

Author

Michel Eduardo Beleza Yamagishi, Nov 06 2024

Status

approved