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A238736 Primes p that divide their Pell quotients, where the Pell quotient of p is A000129(p - (2/p))/p and (2/p) is a Jacobi symbol. 2
13, 31, 1546463 (list; graph; refs; listen; history; text; internal format)



Williams 1982 (p. 86), notes that p = 13, 31 and 1546463 are the only primes less than 10^8 for which the Pell quotient vanishes mod p. Elsenhans and Jahnel, "The Fibonacci sequence modulo p^2," p. 5, report in effect that there are no more such primes p < 10^9.

Williams 1991 (p. 440), and Sun 1995 pt. 3, Theorem 3.3, together prove a set of formulas connecting the Pell quotient with the Fermat quotient (base 2) (A007663) and harmonic numbers like H(floor(p/8)) (see example in the Formula section below). As is well known, the vanishing of the Fermat quotient (base 2) is a necessary condition for the failure of the first case of Fermat's Last Theorem (see discussion under A001220); and in light of a corresponding result of Dilcher and Skula concerning this type of harmonic number, the vanishing of the Pell quotient mod p is also a necessary condition for the failure of the first case of Fermat's Last Theorem.

There are no more terms up to 10^10.

Using the PARI script by Charles R Greathouse IV, I have extended the search from 10^10 to 10^12 without finding a further solution. - John Blythe Dobson, Mar 30 2015


Table of n, a(n) for n=1..3.

Karl Dilcher and Ladislav Skula, A new criterion for the first case of Fermat's Last Theorem, Mathematics of Computation, 64 (1995), 363-392.

Andreas-Stephan Elsenhans and Jörg Jahnel, The Fibonacci sequence modulo p^2 -- An investigation by computer for p < 10^14, arXiv 1006.0824 [math.NT], 2010.

Georges Gras, On the structure of the Galois group of the Abelian closure of a number field, arXiv 1212.3588 [math.NT], 2013.

Hao Pan, Lehmer-type congruences for lacunary harmonic sums, arXiv 0905.0941 [math.NT], 2009.

G. K. Panda and S. S. Rout, Periodicity of Balancing Numbers, Acta Mathematica Hungarica 143 (2014), 274-286. Also on ResearchGate.

Sudhansu Sekhar Rout, Balancing non-Wieferich primes in arithmetic progression and abc conjecture, Proc. Japan Acad. Ser. A Math. Sci., Volume 92, Number 9 (2016), 112-116.

Zhi-Hong Sun, Combinatorial sum ... and its applications in Number Theory, III (English version), originally published in Chinese in Journal of Nanjing University Mathematical Biquarterly, 12 (1995), 90-102.

Zhi-Hong Sun, Five congruences for primes, Fibonacci Quarterly, 40 (2002), 345-351.

H. C. Williams, The influence of computers in the development of number theory, Computers & Mathematics with Applications, 8 (1982), 75-93.

H. C. Williams, Some formulas concerning the fundamental unit of a real quadratic field, Discrete Mathematics, 92 (1991), 431--440.


The condition for p to be a member of this sequence is A000129(p-e)/p == F(p-e, 2)/p == 0 (mod p), where F(p-e, 2) is the p-e'th Fibonacci polynomial evaluated at the argument 2, and e = (2/p) is a Jacobi Symbol.

Let PellQuotient(p) = A000129(p-e)/p, q_2 = (2^(p-1) - 1)/p = A007663(p) be the corresponding Fermat quotient of base 2, H(floor(p/8)) be a harmonic number, and e = (2/p) be a Jacobi Symbol. Then a result of Williams (1991), as refined by Sun (1995), shows that 2*PellQuotient(p) == -4*q_2 - H(floor(p/8)) (mod p).


PellQuotient(13) = 6214 = 13*478; PellQuotient(31) = 3470274850 = 31*111944350.


Select[Prime[Range[1000]], Mod[Fibonacci[# - JacobiSymbol[2, #], 2]/#, #] == 0 &]


(PARI) is(n)=isprime(n) && (Mod([2, 1; 1, 0], n^2)^(n-kronecker(2, n)))[2, 1]==0 \\ Charles R Greathouse IV, Mar 04 2014


Cf. A000129

Sequence in context: A214488 A247836 A159670 * A087511 A299449 A118513

Adjacent sequences:  A238733 A238734 A238735 * A238737 A238738 A238739




John Blythe Dobson, Mar 04 2014



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