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 A002496 Primes of the form n^2 + 1. (Formerly M1506 N0592) 196
 2, 5, 17, 37, 101, 197, 257, 401, 577, 677, 1297, 1601, 2917, 3137, 4357, 5477, 7057, 8101, 8837, 12101, 13457, 14401, 15377, 15877, 16901, 17957, 21317, 22501, 24337, 25601, 28901, 30977, 32401, 33857, 41617, 42437, 44101, 50177 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It is conjectured that this sequence is infinite, but this has never been proved. An equivalent description: primes of form P = (p1*p2*...*pm)^k + 1 where p1..pm are primes and k > 1, since then k must be even for P to be prime. Also prime = p(n) if A054269(n) = 1, i.e., quotient-cycle-length = 1 in continued fraction expansion of sqrt(p). - Labos Elemer, Feb 21 2001 Also primes p such that phi(p) is a square. Also primes of form x*y + z, where x, y and z are three successive numbers. - Giovanni Teofilatto, Jun 05 2004 It is a result that goes back to Mirsky that the set of primes p for which p-1 is squarefree has density A, where A denotes the Artin constant (A = prod_q (1-1/(q(q-1))), q running over all primes). Numerically A = 0.3739558136.... More precisely, Sum_{p <= x} mu(p-1)^2 = Ax/log x + o(x/log x) as x tends to infinity. Conjecture: Sum_{p <= x, mu(p-1)=1} 1 = (A/2)x/log x + o(x/log x) and Sum_{p <= x, mu(p-1)=-1} 1 = (A/2)x/log x + o(x/log x). - Pieter Moree (moree(AT)mpim-bonn.mpg.de), Nov 03 2003 Also primes of the form x^y + 1, where x > 0, y > 1. Primes of the form x^y - 1 (x > 0, y > 1) are the Mersenne primes listed in A000668(n) = {3, 7, 31, 127, 8191, 131071, 524287, 2147483647, ...}. - Alexander Adamchuk, Mar 04 2007 With exception of the first two terms {2,5}, the continued fraction (1 + sqrt(p))/2 has period 3. - Artur Jasinski, Feb 03 2010 With exception of the first term {2}, congruent to 1 (mod 4). - Artur Jasinski, Mar 22 2011 With exception of the first two terms, congruent to 1 or 17 (mod 20). - Robert Israel, Oct 14 2014 From Bernard Schott, Mar 22 2019: (Start) These primes are the primitive terms which generate the sequence of integers with only one prime factor and whose Euler's totient is square: A054755. So this sequence is a subsequence of A054755 and of A039770. Additionally, the terms of this sequence also have a square cototient, so this sequence is a subsequence of A063752 and A054754. If p prime = n^2 + 1, phi(p) = n^2 and cototient(p) = 1^2. Except for 3, the four Fermat primes in A019434 {5, 17, 257, 65537}, belong to this sequence; with F_k = 2^(2^k) + 1, phi(F_k) = (2^(2^(k-1)))^2. See the file "Subfamilies and subsequences" (& I) in A039770 for more details, proofs with data, comments, formulas and examples. (End) REFERENCES J.-M. De Koninck & A. Mercier, 1001 Problèmes en Théorie Classique des Nombres, Problem 211 pp. 34; 169, Ellipses Paris 2004. L. Euler, De numeris primis valde magnis (E283), reprinted in: Opera Omnia. Teubner, Leipzig, 1911, Series (1), Vol. 3, p. 22. G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 5th ed., Oxford Univ. Press, 1979, th. 17. H. L. Montgomery, Ten Lectures on the Interface Between Analytic Number Theory and Harmonic Analysis, Amer. Math. Soc., 1996, p. 208. C. S. Ogilvy, Tomorrow's Math. 2nd ed., Oxford Univ. Press, 1972, p. 116. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). David Wells, The Penguin Dictionary of Curious and Interesting Numbers (Rev. ed. 1997), p. 134 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 T. Amdeberhan, L. A. Median, V. H. Moll, Arithmetical properties of a sequence arising from an arctangent sum, J. Numb. Theory 128 (2008) 1807-1846, eq. (1.10). W. D. Banks, J. B. Friedlander, C. Pomerance and I. E. Shparlinski, Multiplicative structure of values of the Euler function, in High Primes and Misdemeanours: Lectures in Honour of the Sixtieth Birthday of Hugh Cowie Williams (A. Van der Poorten, ed.), Fields Inst. Comm. 41 (2004), pp. 29-47. P. Bateman and R. A. Horn, A heuristic asymptotic formula concerning the distribution of prime numbers, Mathematics of Computation, 16 (1962), 363-367. F. Ellermann, Primes of the form (m^2)+1 up to 10^6 Leon Mirsky, The number of representations of an integer as the sum of a prime and a k-free integer, Amer. Math. Monthly 56 (1949), 17-19. Maxie D. Schmidt, New Congruences and Finite Difference Equations for Generalized Factorial Functions, arXiv:1701.04741 [math.CO], 2017. Eric Weisstein's World of Mathematics, Landau's Problems. Eric Weisstein's World of Mathematics, Near-Square Prime Wikipedia, Bateman-Horn Conjecture Marek Wolf, Search for primes of the form m^2+1, arXiv:0803.1456 [math.NT], 2008. FORMULA There are O(sqrt(n)/log(n)) members of this sequence up to n. But this is just an upper bound. See the Bateman-Horn or Wolf papers, for example, for the conjectured for what is believed to be the correct density. a(n) = 1 + A005574(n)^2. - R. J. Mathar, Jul 31 2015 MAPLE select(isprime, [2, seq(4*i^2+1, i= 1..1000)]); # Robert Israel, Oct 14 2014 MATHEMATICA Select[Range^2+1, PrimeQ] Join[{2}, Select[Range[2, 300, 2]^2+1, PrimeQ]] (* Harvey P. Dale, Dec 18 2018 *) PROG (PARI) isA002496(n) = isprime(n) && issquare(n-1) \\ Michael B. Porter, Mar 21 2010 (PARI) is_A002496(n)=issquare(n-1)&&isprime(n) \\ For "random" numbers in the range 10^10 and beyond, at least 5 times faster than the above. - M. F. Hasler, Oct 14 2014 (MAGMA) [p: p in PrimesUpTo(100000)| IsSquare(p-1)]; // Vincenzo Librandi, Apr 09 2011 (Haskell) a002496 n = a002496_list !! (n-1) a002496_list = filter ((== 1) . a010051') a002522_list -- Reinhard Zumkeller, May 06 2013 (Python) # Python 3.2 or higher required from itertools import accumulate from sympy import isprime A002496_list = [n+1 for n in accumulate(range(10**5), lambda x, y:x+2*y-1) if isprime(n+1)] # Chai Wah Wu, Sep 23 2014 (Python) # Python 2.4 or higher required from sympy import isprime A002496_list = list(filter(isprime, (n*n+1 for n in range(10**5)))) # David Radcliffe, Jun 26 2016 CROSSREFS Cf. A083844 (number of these primes < 10^n), A199401 (growth constant). Cf. A001912, A005574, A054964, A062325, A088179, A090693, A141293. Cf. A000668 (Mersenne primes), A019434 (Fermat primes). Subsequence of A039770. Cf. A010051, subsequence of A002522. Cf. A237040 (an analog for n^3 + 1). Cf. A010051, A000290; subsequence of A028916. Subsequence of A039770, A054754, A054755, A063752. Primes of form n^2+b^4, b fixed: A243451 (b=2), A256775 (b=3), A256776 (b=4), A256777 (b=5), A256834 (b=6), A256835 (b=7), A256836 (b=8), A256837 (b=9), A256838 (b=10), A256839 (b=11), A256840 (b=12), A256841 (b=13). Sequence in context: A078324 A240322 A276460 * A127436 A064168 A118727 Adjacent sequences:  A002493 A002494 A002495 * A002497 A002498 A002499 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS Formula, reference, and comment from Charles R Greathouse IV, Aug 24 2009 Edited by M. F. Hasler, Oct 14 2014 STATUS approved

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Last modified October 14 12:02 EDT 2019. Contains 328004 sequences. (Running on oeis4.)