A377511 a(n) = number of iterations of x -> 2 x + 7 to reach a nonprime, starting with prime(n).

3, 2, 4, 1, 2, 1, 3, 1, 4, 1, 1, 1, 2, 1, 2, 3, 1, 1, 1, 2, 1, 1, 3, 1, 1, 1, 1, 1, 1, 2, 1, 2, 3, 1, 1, 1, 1, 1, 1, 2, 1, 1, 2, 1, 3, 1, 1, 1, 3, 1, 1, 1, 1, 2, 3, 1, 1, 1, 1, 2, 1, 5, 1, 1, 1, 3, 1, 1, 3, 1, 1, 1, 1, 1, 1, 3, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1

Offset

1,1

Comments

See A377120 for a guide to related sequences.

Links

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

Example

prime(3) = 5 -> 17 -> 41 -> 89 -> 185 = 5*37, so a(3) = 4.

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), i=1..100)]);

Mathematica

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

Crossrefs

Cf. A377120, A377510, A377512, A377513, A377514.

Sequence in context: A097673 A226377 A140430 * A123359 A376070 A378423

Adjacent sequences: A377508 A377509 A377510 * A377512 A377513 A377514

Keyword

nonn

Author

Clark Kimberling, Oct 31 2024

Status

approved