A276565 - OEIS

%I #18 Dec 01 2016 04:21:32

%S 56,552,870,1056,1190,1640,1892,2652,4032,5256,5402,6806,8372,9120,

%T 9506,9702,10920,11772,12656,12882,15006,15252,15500,16256,16770,

%U 17556,18632,23256,24492,27722,29070,30800,33306,33672,34410,36290,40200,40602,44310,45582,46872,49506

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

%C Intersection of A002378 and A124936. - _Michel Marcus_, Nov 26 2016

%H Robert Israel, <a href="/A276565/b276565.txt">Table of n, a(n) for n = 1..10000</a>

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

%p 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

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

%Y Cf. A002378, A124936.

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

%K nonn

%O 1,1

%A _Antonio Roldán_, Nov 16 2016