A155875 Smallest positive composite number such that a(n) - n is also composite.

4, 9, 6, 9, 8, 9, 10, 15, 12, 15, 14, 15, 16, 21, 18, 21, 20, 21, 22, 25, 24, 25, 26, 27, 28, 33, 30, 33, 32, 33, 34, 35, 36, 39, 38, 39, 40, 45, 42, 45, 44, 45, 46, 49, 48, 49, 50, 51, 52, 55, 54, 55, 56, 57, 58, 63, 60, 63, 62, 63, 64, 65, 66, 69, 68, 69, 70

Offset

0,1

Comments

a(0) = 4. Subtracting n = 0 from a(0) gives 4-0 = 4, which is a composite number; subtracting n = 1 from a(1) gives 9-1 = 8 which is composite; subtracting n = 2 from a(2) gives 6-2 = 4 which is composite; subtracting n = 3 from a(3) gives 9-3 = 6 which is composite; etc.

Links

Table of n, a(n) for n=0..66.

Prog

(Python)
from sympy import isprime
def composite(n): return n >= 4 and not isprime(n)
def a(n):
  an = n + 4
  while not (composite(an) and composite(an-n)): an += 1
  return an
print([a(n) for n in range(67)]) # Michael S. Branicky, Apr 09 2021

Crossrefs

Cf. A155874, A122985.

Sequence in context: A075635 A362213 A094243 * A105274 A264264 A089390

Adjacent sequences: A155872 A155873 A155874 * A155876 A155877 A155878

Keyword

base,easy,nonn

Author

Eric Angelini, Jan 29 2009

Extensions

More terms from Michael S. Branicky, Apr 09 2021

Status

approved