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A110300
Not-really primes: numbers n such that n - 1, n and n + 1 all have only 1 or 2 prime factors.
1
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
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
Sequence in context: A164977 A103033 A099561 * A143713 A271821 A261459
KEYWORD
easy,nonn
AUTHOR
Walter Nissen, Aug 03 2005
STATUS
approved