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A358128
a(n) is the least semiprime x such that x-2*n and x+2*n are prime.
2
9, 9, 25, 15, 21, 25, 33, 21, 25, 33, 25, 35, 33, 33, 49, 35, 39, 65, 51, 57, 55, 57, 51, 55, 57, 55, 77, 93, 69, 77, 65, 87, 85, 111, 87, 77, 77, 87, 85, 87, 85, 95, 93, 91, 121, 159, 177, 115, 129, 111, 121, 123, 123, 115, 123, 115, 119, 123, 121, 143, 129, 183, 143, 141, 133, 145, 177, 141, 143
OFFSET
1,1
COMMENTS
If n == 0 (mod 3) then a(n) == 1 or 5 (mod 6).
If n == 1 (mod 3) then a(n) == 3 or 5 (mod 6).
If n == 2 (mod 3) then a(n) == 1 or 3 (mod 6).
LINKS
EXAMPLE
a(3) = 25 because 25 = 5^2 is a semiprime and 25 - 2*3 = 19 and 25 + 2*3 = 31 are primes, and 25 is the least semiprime that works.
MAPLE
f:= proc(n) local p;
p:= 1:
do
p:= nextprime(p);
if isprime(p+4*n) and numtheory:-bigomega(p+2*n) = 2 then return p+2*n fi
od
end proc:
map(f, [$1..100]);
PROG
(PARI) a(n) = my(x=2*n+2); while ((bigomega(x)!=2) || !isprime(x-2*n) || !isprime(x+2*n), x++); x; \\ Michel Marcus, Oct 31 2022
CROSSREFS
Cf. A001358.
Sequence in context: A282269 A205380 A246035 * A147340 A147499 A146591
KEYWORD
nonn
AUTHOR
J. M. Bergot and Robert Israel, Oct 30 2022
STATUS
approved