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A175607 Largest number k such that the greatest prime factor of k^2-1 is prime(n). 42
3, 17, 161, 8749, 19601, 246401, 672281, 23718421, 10285001, 354365441, 3222617399, 9447152318, 127855050751, 842277599279, 2218993446251, 2907159732049, 41257182408961, 63774701665793, 25640240468751, 238178082107393, 4573663454608289, 19182937474703818751, 34903240221563713, 332110803172167361, 99913980938200001 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For any prime p, there are finitely many k such that k^2-1 has p as its largest prime factor.

For every prime p, is there some k where the greatest prime factor of k^2-1 is p? Answer from Artur Jasinski, Oct 22 2010: Yes.

As mentioned by Luca and Najman, this problem is closely related to the one in A002071.

The terms give an upper bound with a method for the simultaneous computation of logarithms of small primes, see the fxtbook link. [Joerg Arndt, Jul 03 2012]

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..25

Joerg Arndt, Matters Computational (The Fxtbook), section 32.4, pp.632-633.

Florian Luca and Filip Najman, "On the largest prime factor of x^2-1", Mathematics of Computation 80:273 (2011), pp. 429-435. (Paper has errata that was posted on the MOC website.)

Filip Najman, Home Page (gives all 16167 numbers n such that n^2-1 has no prime factor greater than 97)

PROG

(PARI)  /* up to term for p=97 */

/* S[] is the list computed by Filip Najman (16223 elements) */

S=[2, 3, 4, ... , 332110803172167361, 19182937474703818751];

lpf(n)={ vecmax(factor(n)[, 1]) } /* largest prime factor */

{ forprime (p=2, 97,

  t = 0;

  for (n=1, #S, if ( lpf(S[n]^2-1)==p, t=n ) );

  print1(S[t], ", ");

); }

/* Joerg Arndt, Jul 03 2012 */

CROSSREFS

Cf. A214093 (largest primes p such that the greatest prime factor of p^2-1 is prime(n)).

Cf. A076605 (largest prime divisor of n^2-1).

Sequence in context: A066211 A163884 A221410 * A052143 A069856 A214346

Adjacent sequences:  A175604 A175605 A175606 * A175608 A175609 A175610

KEYWORD

nice,nonn,hard

AUTHOR

Charles R Greathouse IV, Jul 23 2010

EXTENSIONS

More terms (using Filip Najman's list) by Joerg Arndt, Jul 03 2012

STATUS

approved

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Last modified November 23 22:41 EST 2014. Contains 249866 sequences.