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A276565 Oblong numbers n such that n - 1 and n + 1 are both semiprime. 2
56, 552, 870, 1056, 1190, 1640, 1892, 2652, 4032, 5256, 5402, 6806, 8372, 9120, 9506, 9702, 10920, 11772, 12656, 12882, 15006, 15252, 15500, 16256, 16770, 17556, 18632, 23256, 24492, 27722, 29070, 30800, 33306, 33672, 34410, 36290, 40200, 40602, 44310, 45582, 46872, 49506 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Intersection of A002378 and A124936. - Michel Marcus, Nov 26 2016

LINKS

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

EXAMPLE

1640 is oblong (1640 = 40*41) and 1639 = 11*149, 1641 = 3*547 are both semiprime.

MAPLE

select(t -> numtheory:-bigomega(t+1)=2 and numtheory:-bigomega(t-1)=2, [seq(i*(i+1), i=1..1000)]); # Robert Israel, Nov 28 2016

PROG

(PARI) for(i=1, 250, n=i*(i+1); if(bigomega(n-1)==2&&bigomega(n+1)==2, print1(n, ", ")))

CROSSREFS

Cf. A002378, A124936.

Cf. A001597, A108278, A121898, A167023, A268043, A276564.

Sequence in context: A190070 A220930 A220209 * A048540 A189497 A219712

Adjacent sequences:  A276562 A276563 A276564 * A276566 A276567 A276568

KEYWORD

nonn

AUTHOR

Antonio Roldán, Nov 16 2016

STATUS

approved

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Last modified January 23 00:58 EST 2018. Contains 298093 sequences.