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

Author

Walter Nissen, Aug 03 2005

Keywords

• easy
• nonn

Examples

			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.
		

Links

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

Crossrefs

Cf. A000040, A106639.

Programs

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

PARI

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