A121898 - OEIS

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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.

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: A250607 A076579 A345350 * A048190 A090391 A275083

Adjacent sequences: A121895 A121896 A121897 * A121899 A121900 A121901

KEYWORD

nonn

AUTHOR

Jason Earls, Sep 01 2006

STATUS

approved