A327092 Numbers k that are neither prime nor semiprime, and one of n-1 and n+1 is prime while the other is semiprime.

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

Offset

1,1

Links

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

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

Adjacent sequences: A327089 A327090 A327091 * A327093 A327094 A327095

Keyword

nonn

Author

J. M. Bergot and Robert Israel, Sep 13 2019

Status

approved