A109290 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

Other ways to Give

A109290

Composite numbers which are not of the forms p*q -+ 1, where p and q are (not necessarily distinct) primes.

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)

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 A074998 A061715

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