A110300 Not-really primes: numbers n such that n - 1, n and n + 1 all have only 1 or 2 prime factors.

3, 4, 5, 6, 10, 14, 22, 34, 38, 58, 86, 94, 122, 142, 158, 178, 202, 214, 218, 302, 382, 394, 446, 502, 542, 634, 698, 718, 842, 878, 922, 1042, 1138, 1202, 1262, 1318, 1346, 1382, 1402, 1438, 1622, 1642, 1762, 1822, 1838, 1894, 1942, 1982, 2018, 2102

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..6000

Example

14 is in the sequence because 13, 14 and 15 have only 1, 2 and 2 prime factors respectively, viz. 13, 2 * 7 and 3 * 5.

Maple

S:= select(t -> numtheory:-bigomega(t) <= 2, {$2..3000}):
sort(convert(S intersect map(`+`, S, 1) intersect map(`-`, S, 1), list)); # Robert Israel, Dec 15 2019

Prog

(PARI) isok(n) = (bigomega(n-1)>0) && (bigomega(n-1)<=2) && (bigomega(n)<=2) && (bigomega(n+1)<=2) \\ Michel Marcus, Jul 23 2013

Crossrefs

Cf. A000040, A106639.

Sequence in context: A164977 A103033 A099561 * A143713 A271821 A261459

Adjacent sequences: A110297 A110298 A110299 * A110301 A110302 A110303

Keyword

easy,nonn

Author

Walter Nissen, Aug 03 2005

Status

approved