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A109290 Composite numbers which are not of the forms p*q -+ 1, where p and q are (not necessarily distinct) primes. 4
4, 6, 12, 18, 28, 30, 42, 44, 46, 49, 51, 55, 60, 62, 65, 69, 72, 74, 77, 80, 82, 91, 98, 99, 100, 102, 104, 106, 108, 111, 115, 125, 126, 129, 136, 138, 148, 150, 152, 153, 155, 161, 164, 166, 169, 171, 172, 174, 175, 180, 183, 185, 187, 189, 190, 192, 194, 196 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

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

MAPLE

filter:= proc(n)

   if n::even then numtheory:-bigomega(n+1) <> 2 and numtheory:-bigomega(n-1) <> 2

   elif n mod 4 = 1 then not isprime(n) and not isprime((n+1)/2)

   else not isprime(n) and not isprime((n-1)/2)

   fi

end proc:

select(filter, [$4..200]); # Robert Israel, Apr 20 2021

MATHEMATICA

bo[n_] := Plus @@ Last /@ FactorInteger[n]; Select[Range[2, 200], ! (PrimeQ[ # ] || bo[ # - 1] == 2 || bo[ # + 1] == 2) &] (* Ray Chandler, Aug 27 2005 *)

CROSSREFS

Cf. A109287, A109288, A109289.

Sequence in context: A072568 A154965 A249630 * A130441 A068570 A074998

Adjacent sequences:  A109287 A109288 A109289 * A109291 A109292 A109293

KEYWORD

nonn

AUTHOR

Giovanni Teofilatto, Aug 20 2005

EXTENSIONS

Corrected and extended by Ray Chandler, Aug 27 2005

STATUS

approved

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Last modified July 23 14:38 EDT 2021. Contains 346259 sequences. (Running on oeis4.)