login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A291199 Primes p such that phi(p*(p+1)/2) is a triangular number (A000217). 1
2477, 44287823, 58192759, 110369351, 664009019, 2574106333, 6870260119, 7423240007, 60370077539, 188271042191, 235399729007, 236767359977, 305214702643, 717724689959 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(15) > 10^12. - Giovanni Resta, Aug 21 2017

LINKS

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

EXAMPLE

Prime number 2477 is a term since phi(2477*2478/2) = 1856*1857/2.

PROG

(PARI) isok(n) = isprime(n) && ispolygonal(eulerphi(n*(n+1)/2), 3);

(PARI) is(n) = ispolygonal(eulerphi(n\2+1)*(n-1), 3) && isprime(n) \\ Charles R Greathouse IV, Aug 22 2017

(Python)

from __future__ import division

from sympy.ntheory.primetest import is_square

from sympy import totient, nextprime

A291199_list, p = [], 3

while p < 10**8:

    if is_square(8*(p-1)*totient((p+1)//2)+1):

        A291199_list.append(p)

    p = nextprime(p) # Chai Wah Wu, Aug 22 2017

CROSSREFS

Cf. A000010, A000217, A034953, A086700.

Sequence in context: A020411 A124594 A241048 * A260766 A003917 A235763

Adjacent sequences:  A291196 A291197 A291198 * A291200 A291201 A291202

KEYWORD

nonn,more

AUTHOR

Altug Alkan, Aug 20 2017

EXTENSIONS

a(5)-a(14) from Giovanni Resta, Aug 21 2017

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 21 07:23 EDT 2019. Contains 321367 sequences. (Running on oeis4.)