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 A291340 Primes p such that phi(p*(p-1)/2) is a triangular number (A000217). 0
 2, 617, 11243, 72817, 134213, 1083601, 1301617, 669175321, 2759347757, 5023245823, 10904836391, 11230923643, 12828977503, 27741297781 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Inspired by A291199. LINKS PROG (Python) from __future__ import division from sympy.ntheory.primetest import is_square from sympy import totient, nextprime A291340_list, p = [2], 3 while p < 10**6:     if is_square(8*(p-1)*totient((p-1)//2)+1):         A291340_list.append(p)     p = nextprime(p) CROSSREFS Cf. A000217, A291199. Sequence in context: A281537 A134796 A120830 * A332162 A046856 A156938 Adjacent sequences:  A291337 A291338 A291339 * A291341 A291342 A291343 KEYWORD nonn,more AUTHOR Chai Wah Wu, Aug 31 2017 STATUS approved

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Last modified May 20 13:32 EDT 2022. Contains 353873 sequences. (Running on oeis4.)