A121898 Triangular numbers that are sandwiched between two semiprimes; or triangular numbers t such that t-1 and t+1 are both semiprime.

120, 300, 528, 780, 2628, 3240, 3828, 5460, 13530, 18528, 19110, 22578, 25878, 31878, 32640, 37128, 49770, 56280, 64980, 72390, 73920, 78210, 103740, 105570, 115440, 137550, 159330, 161028, 277140, 288420, 316410, 335790, 370230, 386760

Offset: 1

Author

Jason Earls, Sep 01 2006

Keywords

• nonn

Comments

Why are so many of the t-1's brilliant numbers (A078972)?

Programs

Mathematica

Select[Accumulate[Range[1000]],PrimeOmega[#-1]==PrimeOmega[#+1]==2&] (* Harvey P. Dale, Jul 06 2014 *)