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A155875
Smallest positive composite number such that a(n) - n is also composite.
1
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.
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
Sequence in context: A075635 A362213 A094243 * A105274 A264264 A089390
KEYWORD
base,easy,nonn
AUTHOR
Eric Angelini, Jan 29 2009
EXTENSIONS
More terms from Michael S. Branicky, Apr 09 2021
STATUS
approved