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A121898 Triangular numbers that are sandwiched between two semiprimes; or triangular numbers t such that t-1 and t+1 are both semiprime. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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

LINKS

Table of n, a(n) for n=1..34.

MATHEMATICA

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

CROSSREFS

Sequence in context: A257712 A250607 A076579 * A048190 A090391 A275083

Adjacent sequences:  A121895 A121896 A121897 * A121899 A121900 A121901

KEYWORD

nonn

AUTHOR

Jason Earls, Sep 01 2006

STATUS

approved

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Last modified October 21 01:18 EDT 2019. Contains 328291 sequences. (Running on oeis4.)