A276565 Oblong numbers n such that n - 1 and n + 1 are both semiprime.

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

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