
OFFSET

1,1


COMMENTS

Sequence is believed to be infinite.
Joseph Silverman showed that the abcconjecture implies that there are infinitely many primes which are not in the sequence.  Benoit Cloitre, Jan 09 2003
Graves and Murty (2013) improved Silverman's result by showing that for any fixed k > 1, the abcconjecture implies that there are infinitely many primes == 1 (mod k) which are not in the sequence.  Jonathan Sondow, Jan 21 2013
The squares of these numbers are Fermat pseudoprimes to base 2 (A001567) and Catalan pseudoprimes (A163209).  T. D. Noe, May 22 2003
Primes p that divide the numerator of the harmonic number H((p1)/2); that is, p divides A001008((p1)/2).  T. D. Noe, Mar 31 2004
In a 1977 paper, Wells Johnson, citing a suggestion from Lawrence Washington, pointed out the repetitions in the binary representations of the numbers which are one less than the two known Wieferich primes; i.e., 1092 = 10001000100 (base 2); 3510 = 110110110110 (base 2). It is perhaps worth remarking that 1092 = 444 (base 16) and 3510 = 6666 (base 8), so that these numbers are small multiples of repunits in the respective bases. Whether this is mathematically significant does not appear to be known.  John Blythe Dobson, Sep 29 2007
A002326((a(n)^2  1)/2) = A002326((a(n)1)/2).  Vladimir Shevelev, Jul 09 2008, Aug 24 2008
It is believed that p^2 does not divide 3^(p1)  1 if p = a(n). This is true for n = 1 and 2. See A178815, A178844, A178900, and OstafeShparlinski (2010) Section 1.1.  Jonathan Sondow, Jun 29 2010
These primes also divide the numerator of the harmonic number H(floor((p1)/4)).  H. Eskandari (hamid.r.eskandari(AT)gmail.com), Sep 28 2010
1093 and 3511 are prime numbers p satisfying congruence 429327^(p1) == 1 (mod p^2). Why?  Arkadiusz Wesolowski, Apr 07 2011. Such bases are listed in A247208.  Max Alekseyev, Nov 25 2014. See A269798 for all such bases, prime and composite, that are not powers of 2.  Felix Fröhlich, Apr 07 2018
A196202(A049084(a(1)) = A196202(A049084(a(2)) = 1.  Reinhard Zumkeller, Sep 29 2011
If q is prime and q^2 divides a primeexponent Mersenne number, then q must be a Wieferich prime. Neither of the two known Wieferich primes divide Mersenne numbers. See Will Edgington's Mersenne page in the links below.  Daran Gill, Apr 04 2013
There are no other terms below 4.97*10^17 as established by PrimeGrid (see link below).  Max Alekseyev, Nov 20 2015. The search was done via PrimeGrid's PRPNet and the results were not doublechecked. Because of the unreliability of the testing, the search was suspended in May 2017 (cf. Goetz, 2017).  Felix Fröhlich, Apr 01 2018
Are there other primes q >= p such that q^2 divides 2^(p1)1, where p is a prime?  Thomas Ordowski, Nov 22 2014. Any such q must be a Wieferich prime.  Max Alekseyev, Nov 25 2014
Primes p such that p^2 divides 2^r  1 for some r, 0 < r < p.  Thomas Ordowski, Nov 28 2014, corrected by Max Alekseyev, Nov 28 2014
For some reason, both p=a(1) and p=a(2) also have more bases b with 1<b<p that make b^(p1)==1 (mod p^2) than any smaller prime p; in other words a(1) and a(2) belong to A248865.  Jeppe Stig Nielsen, Jul 28 2015
Let r_1, r_2, r_3, ...., r_i be the set of roots of the polynomial X^((p1)/2)  (p3)! * X^((p3)/2)  (p5)! * X^((p5)/2)  ...  1. Then p is a Wieferich prime iff p divides sum{k=1, p}(r_k^((p1)/2)) (see Example 2 in Jakubec, 1994).  Felix Fröhlich, May 27 2016
Arthur Wieferich showed that if p is not a term of this sequence, then the First Case of Fermat's Last Theorem has no solution in x, y and z for prime exponent p (cf. Wieferich, 1909).  Felix Fröhlich, May 27 2016
Let U_n(P, Q) be a Lucas sequence of the first kind, let e be the Legendre symbol (D/p) and let p be a prime not dividing 2QD, where D = P^2  4*Q. Then a prime p such that U_(pe) == 0 (mod p^2) is called a "LucasWieferich prime associated to the pair (P, Q)". Wieferich primes are those LucasWieferich primes that are associated to the pair (3, 2) (cf. McIntosh, Roettger, 2007, p. 2088).  Felix Fröhlich, May 27 2016
Any repeated prime factor of a term of A000215 is a term of this sequence. Thus, if there exist infinitely many Fermat numbers that are not squarefree, then this sequence is infinite, since no two Fermat numbers share a common factor.  Felix Fröhlich, May 27 2016
If the Diophantine equation p^x  2^y = d has more than one solution in positive integers (x, y), with (p, d) not being one of the pairs (3, 1), (3, 5), (3, 13) or (5, 3), then p is a term of this sequence (cf. Scott, Styer, 2004, Corollary to Theorem 2).  Felix Fröhlich, Jun 18 2016
Odd primes p such that Chi_(D_0)(p) != 1 and Lambda_p(Q(sqrt(D_0))) != 1, where D_0 < 0 is the fundamental discriminant of the imaginary quadratic field Q(sqrt(1p^2)) and Chi and Lambda are Iwasawa invariants (cf. Byeon, 2006, Proposition 1 (i)).  Felix Fröhlich, Jun 25 2016
If q is an odd prime, k, p are primes with p = 2*k+1, k == 3 (mod 4), p == 1 (mod q) and p =/= 1 (mod q^3) (Jakubec, 1998, Corollary 2 gives p == 5 (mod q) and p =/= 5 (mod q^3)) with the multiplicative order of q modulo k = (k1)/2 and q dividing the class number of the real cyclotomic field Q(Zeta_p + (Zeta_p)^(1)), then q is a term of this sequence (cf. Jakubec, 1995, Theorem 1).  Felix Fröhlich, Jun 25 2016
From Felix Fröhlich, Aug 06 2016: (Start)
Primes p such that p1 is in A240719.
Prime terms of A077816 (cf. Agoh, Dilcher, Skula, 1997, Corollary 5.9).
p = prime(n) is in the sequence iff T(2, n) > 1, where T = A258045.
p = prime(n) is in the sequence iff an integer k exists such that T(n, k) = 2, where T = A258787. (End)
Conjecture: an integer n > 1 such that n^2 divides 2^(n1)1 must be a Wieferich prime.  Thomas Ordowski, Dec 21 2016
The above conjecture is equivalent to the statement that no "Wieferich pseudoprimes" (WPSPs) exist. While baseb WPSPs are known to exist for several bases b > 1 other than 2 (see for example A244752), no base2 WPSPs are known. Since two necessary conditions for a composite to be a base2 WPSP are that, both, it is a base2 Fermat pseudoprime (A001567) and all its prime factors are Wieferich primes (cf. A270833), as shown in the comments in A240719, it seems that the first base2 WPSP, if it exists, is probably very large. This appears to be supported by the guess that the properties of a composite to be a term of A001567 and of A270833 are "independent" of each other and by the observation that the scatterplot of A256517 seems to become "less dense" at the xaxis parallel line y = 2 for increasing n. It has been suggested in the literature that there could be asymptotically about log(log(x)) Wieferich primes below some number x, which is a function that grows to infinity, but does so very slowly. Considering the above constraints, the number of WPSPs may grow even more slowly, suggesting any such number, should it exist, probably lies far beyond any bound a bruteforce search could reach in the forseeable future. Therefore I guess that the conjecture may be false, but a disproof or the discovery of a counterexample are probably extraordinarily difficult problems.  Felix Fröhlich, Jan 18 2019


REFERENCES

R. Crandall and C. Pomerance, Prime Numbers: A Computational Perspective, Springer, NY, 2001; see p. 28.
R. K. Guy, Unsolved Problems in Number Theory, A3.
G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 5th ed., Oxford Univ. Press, 1979, th. 91.
Y. Hellegouarch, "Invitation aux mathématiques de Fermat Wiles", Dunod, 2eme Edition, pp. 340341.
Pace Nielsen, Wieferich primes, heuristics, computations, Abstracts Amer. Math. Soc., 33 (#1, 20912), #10771148.
P. Ribenboim, The Book of Prime Number Records. SpringerVerlag, NY, 2nd ed., 1989, p. 263.
D. Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, 163.


LINKS

Table of n, a(n) for n=1..2.
T. Agoh, K. Dilcher, L. Skula, Fermat Quotients for Composite Moduli, Journal of Number Theory 66(1), 1997, 2950.
Joerg Arndt, Matters Computational (The Fxtbook), p. 780.
Alex Samuel Bamunoba, A note on Carlitz Wieferich primes, Journal of Number Theory 174 (2017) 343357; http://dx.doi.org/10.1016/j.jnt.2016.09.036
D. Byeon, Class numbers, Iwasawa invariants and modular forms, Trends in Mathematics, Vol. 9, No. 1, (2006), 2529.
Chris K. Caldwell, The Prime Glossary, Wieferich prime
Chris K. Caldwell, Primesquare Mersenne divisors are Wieferich
D. X. Charles, On Wieferich Primes
R. Crandall, K. Dilcher and C. Pomerance, A search for Wieferich and Wilson primes, Mathematics of Computation, Volume 66, 1997.
J. K. Crump, Joe's Number Theory Web, Weiferich Primes (sic)
John Blythe Dobson, A note on the two known Wieferich Primes
J. B. Dobson A Characterization of WilsonLerch Primes, Integers, 16 (2016), A51.
F. G. Dorais, WPSE  A Wieferich Prime Search Engine (A program to search Wieferich primes written by F. G. Dorais.)  Felix Fröhlich, Jul 13 2014
F. G. Dorais and D. W. Klyve, A Wieferich Prime Search up to 6.7*10^15, Journal of Integer Sequences, Vol. 14, 2011.
Bruno Dular, Cycles of Sums of Integers, arXiv:1905.01765 [math.NT], 2019.
Will Edgington, Mersenne Page> [from Internet Archive Wayback Machine].
M. Goetz, WSS and WFS are suspended, PrimeGrid forum, Message 107809, May 11, 2017.
A. Granville, K. Soundararajan, A binary additive problem of Erdos and the order of 2 mod p^2, Raman. J. 2 (1998) 283298
Hester Graves and M. Ram Murty, The abc conjecture and nonWieferich primes in arithmetic progressions, Journal of Number Theory, 133 (2013), 18091813.
René Gy, Extended Congruences for Harmonic Numbers, arXiv:1902.05258 [math.NT], 2019.
Lorenz Halbeisen and Norbert Hungerbuehler, Number theoretic aspects of a combinatorial function, Notes on Number Theory and Discrete Mathematics 5 (1999) 138150. (ps, pdf)
S. Jakubec, Connection between the Wieferich congruence and divisibility of h+, Acta Arithmetica, Vol. 71, No. 1 (1995), 5564.
S. Jakubec, On divisibility of the class number h+ of the real cyclotomic fields of prime degree l, Mathematics of Computation, Vol. 67, No. 221 (1998), 369398.
S. Jakubec, The Congruence for Gauss Period, Journal of Number Theory, Vol. 48, No. 1 (1994), 3645.
Wells Johnson, On the nonvanishing of Fermat quotients (mod p), Journal f. die reine und angewandte Mathematik 292, (1977): 196200.
J. Knauer and J. Richstein, The continuing search for Wieferich primes, Math. Comp., 75 (2005), 15591563.
D. H. Lehmer, On Fermat's quotient, base two, Math. Comp. 36 (1981), 289290.
R. J. McIntosh and E. L. Roettger, A search for FibonacciWieferich and Wolstenholme primes, Math. Comp. vol 76, no 260 (2007) pp 20872094.
C. McLeman, PlanetMath.org, Wieferich prime
W. Meissner, Über die Teilbarkeit von 2^p2 durch das Quadrat der Primzahl p = 1093, Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften Berlin, 35 (1913), 663667. [Annotated scanned copy]
Sihem Mesnager and JeanPierre Flori, A note on hyperbent functions via Dillonlike exponents, IACR, Report 2012/033, 2012.
Mishima Miwako and Koji Momihara, A new series of optimal tight conflictavoiding codes of weight 3, Discrete Mathematics 340.4 (2017): 617629. See page 618.
A. Ostafe and I. Shparlinski, Pseudorandomness and Dynamics of Fermat Quotients, arXiv:1001.1504 [math.NT], 2010.
Christian Perfect, Integer sequence reviews on Numberphile (or vice versa), 2013.
R. Scott and R. Styer, On p^x  q^y = c and related three term exponential Diophantine equations with prime bases, Journal of Number Theory, Vol. 105, No. 2 (2004), 212234.
V. Shevelev, Overpseudoprimes, Mersenne Numbers and Wieferich Primes, arXiv:0806.3412 [math.NT], 2008.
J. Silverman, Wieferich's Criterion and the abc Conjecture, J. Number Th. 30 (1988) 226237.
J. Sondow, Lerch quotients, Lerch primes, FermatWilson quotients, and the WieferichnonWilson primes 2, 3, 14771, arXiv:1110.3113 [math.NT], 2012.
J. Sondow, Lerch Quotients, Lerch Primes, FermatWilson Quotients, and the WieferichnonWilson Primes 2, 3, 14771, Combinatorial and Additive Number Theory, CANT 2011 and 2012, Springer Proc. in Math. & Stat., vol. 101 (2014), pp. 243255.
Michel Waldschmidt, Lecture on the abc conjecture and some of its consequences, Abdus Salam School of Mathematical Sciences (ASSMS), Lahore, 6th World Conference on 21st Century Mathematics 2013.
Michel Waldschmidt, Lecture on the abc conjecture and some of its consequences, Abdus Salam School of Mathematical Sciences (ASSMS), Lahore, 6th World Conference on 21st Century Mathematics 2013.
Eric Weisstein's World of Mathematics, Wieferich Prime
Eric Weisstein's World of Mathematics, abc Conjecture
Eric Weisstein's World of Mathematics, Integer Sequence Primes
A. Wieferich, Zum letzten Fermat'schen Theorem, Journal für die reine und angewandte Mathematik, 136 (1909), 293302.
Wikipedia, Wieferich prime
P. Zimmermann, Records for Prime Numbers


FORMULA

A178815(A000720(p))^(p1)  1 mod p^2 = A178900(n), where p = a(n).  Jonathan Sondow, Jun 29 2010
Odd primes p such that A002326((p^21)/2) = A002326((p1)/2). See A182297.  Thomas Ordowski, Feb 04 2014


MAPLE

wieferich := proc (n) local nsq, remain, bin, char: if (not isprime(n)) then RETURN("not prime") fi: nsq := n^2: remain := 2: bin := convert(convert(n1, binary), string): remain := (remain * 2) mod nsq: bin := substring(bin, 2..length(bin)): while (length(bin) > 1) do: char := substring(bin, 1..1): if char = "1" then remain := (remain * 2) mod nsq fi: remain := (remain^2) mod nsq: bin := substring(bin, 2..length(bin)): od: if (bin = "1") then remain := (remain * 2) mod nsq fi: if remain = 1 then RETURN ("Wieferich prime") fi: RETURN ("nonWieferich prime"): end: # Ulrich Schimke (ulrschimke(AT)aol.com), Nov 01 2001


MATHEMATICA

Select[Prime[Range[50000]], Divisible[2^(#1)1, #^2]&] (* Harvey P. Dale, Apr 23 2011 *)
Select[Prime[Range[50000]], PowerMod[2, #1, #^2]==1&] (* Harvey P. Dale, May 25 2016 *)


PROG

(Haskell)
import Data.List (elemIndices)
a001220 n = a001220_list !! (n1)
a001220_list = map (a000040 . (+ 1)) $ elemIndices 1 a196202_list
 Reinhard Zumkeller, Sep 29 2011
(PARI)
N=10^9; default(primelimit, N);
forprime(n=2, N, if(Mod(2, n^2)^(n1)==1, print1(n, ", ")));
\\ Joerg Arndt, May 01 2013
(Python)
from sympy import prime
from gmpy2 import powmod
A001220_list = [p for p in (prime(n) for n in range(1, 10**7)) if powmod(2, p1, p*p) == 1]
# Chai Wah Wu, Dec 03 2014
(GAP) Filtered([1..50000], p>IsPrime(p) and (2^(p1)1) mod p^2 =0); # Muniru A Asiru, Apr 03 2018
(MAGMA) [p : p in PrimesUpTo(310000)  IsZero((2^(p1)  1) mod (p^2))]; // Vincenzo Librandi, Jan 19 2019


CROSSREFS

See A007540 for a similar problem.
Sequences "primes p such that p^2 divides X^(p1)1": A014127 (X=3), A123692 (X=5), A212583 (X=6), A123693 (X=7), A045616 (X=10), A111027 (X=12), A234810 (X=14), A242741 (X=15), A244260 (X=18), A242982 (X=20).
Cf. A001567, A002323, A077816, A001008, A039951, A049094, A126196, A126197, A178815, A178844, A178871, A178900, A246503, A247208, A269798.
Sequence in context: A246503 A077816 A291961 * A265630 A291194 A270833
Adjacent sequences: A001217 A001218 A001219 * A001221 A001222 A001223


KEYWORD

nonn,hard,bref,nice,more


AUTHOR

N. J. A. Sloane


STATUS

approved

