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A158891 Numbers n with property that (2/(n+1))*(Concatenated triangular numbers T (n), T(n+1)) is prime. 0
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 (list; graph; refs; listen; history; text; internal format)
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: A112072 A024902 A111312 * A213220 A087680 A249333

Adjacent sequences:  A158888 A158889 A158890 * A158892 A158893 A158894

KEYWORD

base,nonn

AUTHOR

Zak Seidov, Mar 29 2009

STATUS

approved

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Last modified May 9 13:42 EDT 2021. Contains 343742 sequences. (Running on oeis4.)