A158891 Numbers n with property that (2/(n+1))*(Concatenated triangular numbers T (n), T(n+1)) is prime.

1, 7, 9, 15, 21, 31, 37, 39, 61, 75, 91, 115, 117, 121, 129, 141, 147, 151, 169, 171, 187, 195, 201, 217, 241, 249, 285, 301, 319, 339, 357, 399, 417, 421, 435, 457, 477, 481, 487, 499, 507, 529, 567, 577, 579, 595, 607, 621, 627, 631, 645, 691, 711, 715, 717

Offset

1,2

Comments

All terms are odd. No term == 3 mod 10.

Links

Table of n, a(n) for n=1..55.

Example

n=1: T(1)=1, T(2)=3, (2/2)*13=13 (prime),
n=7: T(7)=28, T(8)=36, (2/8)*2836=709 (prime),
n=9: T(9)=45, T(8)=55, (2/10)*4555=911 (prime).

Crossrefs

A000217 Triangular numbers.

Sequence in context: A024902 A111312 A357500 * A213220 A087680 A371303

Adjacent sequences: A158888 A158889 A158890 * A158892 A158893 A158894

Keyword

base,nonn

Author

Zak Seidov, Mar 29 2009

Status

approved