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%I #5 Dec 15 2017 17:37:06
%S 120,300,528,780,2628,3240,3828,5460,13530,18528,19110,22578,25878,
%T 31878,32640,37128,49770,56280,64980,72390,73920,78210,103740,105570,
%U 115440,137550,159330,161028,277140,288420,316410,335790,370230,386760
%N Triangular numbers that are sandwiched between two semiprimes; or triangular numbers t such that t-1 and t+1 are both semiprime.
%C Why are so many of the t-1's brilliant numbers (A078972)?
%t Select[Accumulate[Range[1000]],PrimeOmega[#-1]==PrimeOmega[#+1]==2&] (* _Harvey P. Dale_, Jul 06 2014 *)
%K nonn
%O 1,1
%A _Jason Earls_, Sep 01 2006