A255200 Numbers n such that triangular numbers T(n), T(n+1) and T(n+2) are 3-almost primes.

17, 28, 41, 281, 2081, 2801, 4721, 5441, 6196, 12161, 12916, 13996, 20476, 24916, 32321, 32441, 34156, 34961, 40036, 56596, 70996, 73361, 94396

Offset

1,1

Comments

Numbers n such that n, n+1 and n+2 are terms in A108815.

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Example

T(17)=A000217(17)=153=3*3*17, T(18)=A000217(18)=171=3*3*19, T(19)=A000217(19)=171=2*5*19.

Prog

(PARI) issemi(n)=bigomega(n)==2
is(n)=if(n%2, if(n%4==1, isprime((n+3)/4) && isprime(n+2) && isprime(n) && issemi((n+1)/2), isprime((n+1)/4) && isprime(n) && isprime(n+2) && issemi((n+3)/2)), if(n%4==2, isprime((n+2)/4) && isprime(n+1) && isprime(n+3) && issemi(n/2), isprime(n/4) && isprime(n+1) && isprime(n+3) && issemi(n/2+1))) \\ Charles R Greathouse IV, Feb 05 2017

Crossrefs

Cf. A000217, A075875, A108815.

Sequence in context: A134468 A366963 A032611 * A350101 A360972 A319265

Adjacent sequences: A255197 A255198 A255199 * A255201 A255202 A255203

Keyword

nonn

Author

Zak Seidov, Feb 16 2015

Status

approved