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A078699 Primes p such that p^2-1 is a triangular number. 1
2, 11, 23, 373, 12671, 901273, 19472752251611, 53072032161200090602953513048447623, 5027153581127740201460650182713355379768873, 11604855412241025458500993236724193227031777965785837784548351709747881343573 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Equivalently, primes in A006452.

The sequence of corresponding triangular numbers begins 3, 120, 528, 139128, 160554240, 812293020528, 379188080252621270252095320, ... [Shreevatsa R, Jul 12 2013]

LINKS

Joerg Arndt, Table of n, a(n) for n = 1..14

MATHEMATICA

a[n_] := a[n]=If[n<4, {1, 2, 4, 11}[[n+1]], 6a[n-2]-a[n-4]]; Select[a/@Range[200], ProvablePrimeQ] (* First do <<NumberTheory`PrimeQ` *)

PROG

(PARI) default(primelimit, 10^7) istri(n) = t=floor(sqrt(2*n)); if(2*n==t*(t+1), 1, 0) forprime(p=2, 5*10^6, if(istri(p^2-1), print1(p" ")))

(PARI) istriang(n)=issquare(8*n+1);

forprime(p=2, 10^10, if(istriang(p^2-1), print1(p, ", ")));

\\ Joerg Arndt, Jul 15 2013

(PARI)  /* much more efficient: */

N=1166; f=( 1+x-4*x^2-2*x^3 ) / ( (x^2+2*x-1)*(x^2-2*x-1) )+O(x^N);

for(n=0, N-1, my(c=polcoeff(f, n)); if(isprime(c), print1(c, ", ")));

\\ Joerg Arndt, Jul 15 2013

CROSSREFS

Cf. A000217, A006452.

Sequence in context: A218046 A217309 A115374 * A291679 A239741 A042347

Adjacent sequences:  A078696 A078697 A078698 * A078700 A078701 A078702

KEYWORD

nonn

AUTHOR

Jason Earls, Dec 18 2002

EXTENSIONS

Edited by Dean Hickerson, Dec 19 2002

STATUS

approved

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Last modified January 22 18:06 EST 2019. Contains 319365 sequences. (Running on oeis4.)