A377514 a(n) = number of iterations of x -> 2 x - 7 to reach a nonprime, starting with prime(n+4).

1, 3, 1, 2, 1, 1, 1, 3, 1, 3, 1, 1, 1, 1, 2, 1, 3, 2, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 1, 3, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1, 3, 1, 1, 1, 1, 2, 1, 1, 4, 3, 2, 1, 1, 3, 1, 2, 1, 1, 1, 2, 1, 1, 1, 2, 1, 3

Offset

1,2

Comments

See A377120 for a guide to related sequences.

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

prime(6) = 13 -> 19 -> 31 -> 55 = 5*11, so a(2) = 3.

Maple

f:= proc(p) local x, i;
  x:= p;
  for i from 1 do
    x:= 2*x-7;
    if not isprime(x) then return i fi;
  od
end proc:
map(f, [seq(ithprime(i+4), i=1..100)]); # Robert Israel, Nov 17 2025

Mathematica

Table[p = Prime[n + 4]; c = 1; While[p = 2*p - 7; PrimeQ[p], c++]; c, {n, 200}]

Crossrefs

Cf. A377120, A377510, A377511, A377512, A377513.

Sequence in context: A375082 A243977 A328569 * A387412 A016569 A072801

Adjacent sequences: A377511 A377512 A377513 * A377515 A377516 A377517

Keyword

nonn

Author

Clark Kimberling, Nov 05 2024

Status

approved