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A263990 Nonsquare numbers k such that k and k + 1 are semiprimes. 7
14, 21, 33, 34, 38, 57, 85, 86, 93, 94, 118, 122, 133, 141, 142, 145, 158, 177, 201, 202, 205, 213, 214, 217, 218, 253, 298, 301, 302, 326, 334, 381, 393, 394, 445, 446, 453, 481, 501, 514, 526, 537, 542, 553, 565, 622, 633, 634, 694, 697, 698, 706, 717, 745, 766, 778, 793, 802, 817, 842, 865, 878 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n and n+1 are semiprimes then n+1 is always nonsquare while n can be a full square (see A263951). The sequence gives the nonsquare terms of A070552. Each of the numbers n and n+1 is a product of two distinct primes.

Numbers that are terms in A070552 but not in A263951.

The subsequence of triples of consecutive squarefree semiprimes is A039833. - R. J. Mathar, Aug 13 2019

LINKS

Seiichi Manyama, Table of n, a(n) for n = 1..10000

MATHEMATICA

Select[Range[1000], ! IntegerQ[Sqrt[#]] && 2 == PrimeOmega[#] == PrimeOmega[# + 1] &]

PROG

(PARI) is(n)=if(n%2, isprime((n+1)/2) && bigomega(n)==2 && !isprimepower(n), isprime(n/2) && bigomega(n+1)==2) \\ Charles R Greathouse IV, Apr 25 2016

CROSSREFS

Subsequence of A070552, A086263.

Cf. A006881, A263951.

Sequence in context: A120141 A240593 A121495 * A253810 A084995 A100466

Adjacent sequences:  A263987 A263988 A263989 * A263991 A263992 A263993

KEYWORD

nonn

AUTHOR

Zak Seidov, Oct 31 2015

STATUS

approved

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Last modified October 18 12:18 EDT 2019. Contains 328160 sequences. (Running on oeis4.)