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A327092 Numbers k that are neither prime nor semiprime, and one of n-1 and n+1 is prime while the other is semiprime. 1
8, 16, 20, 24, 32, 36, 40, 48, 52, 54, 66, 68, 70, 78, 84, 88, 90, 96, 110, 112, 114, 128, 130, 132, 140, 156, 162, 168, 182, 200, 210, 212, 222, 234, 238, 250, 252, 258, 264, 268, 292, 294, 306, 308, 310, 318, 330, 336, 338, 354, 360, 366, 372, 378, 380, 390, 396, 402, 408 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
EXAMPLE
a(3)=20 is a member of the sequence because 20 is neither prime nor semiprime, 20-1=19 is prime and 20+1=21 is semiprime.
MAPLE
issp:= proc(n) numtheory:-bigomega(n)=2 end proc:
filter:= proc(n)
if isprime(n) or issp(n) then return false fi;
if isprime(n-1) then issp(n+1)
else isprime(n+1) and issp(n-1)
fi;
end proc:
select(filter, [seq(i, i=2..1000, 2)]);
CROSSREFS
Cf. A001358.
Sequence in context: A029522 A033309 A114435 * A112679 A212021 A096912
KEYWORD
nonn
AUTHOR
J. M. Bergot and Robert Israel, Sep 13 2019
STATUS
approved

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Last modified April 12 10:04 EDT 2024. Contains 371627 sequences. (Running on oeis4.)