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 A001783 n-phi-torial, or phi-torial of n: Product k, 1 <= k <= n, k relatively prime to n. (Formerly M0921 N0346) 36
 1, 1, 2, 3, 24, 5, 720, 105, 2240, 189, 3628800, 385, 479001600, 19305, 896896, 2027025, 20922789888000, 85085, 6402373705728000, 8729721, 47297536000, 1249937325, 1124000727777607680000, 37182145, 41363226782215962624 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS In other words, a(1) = 1 and for n >= 2, a(n) = product of the phi(n) numbers < n and relatively prime to n. From Gauss's generalization of Wilson's theorem (see Weisstein link) it follows that, for n>1, a(n) == -1 (mod n) if and only if there exists a primitive root modulo n (cf. the Hardy and Wright reference, Theorem 129. p. 102). - Vladimir Shevelev, May 11 2012 Islam & Manzoor prove that a(n) is an injection for n > 1, see links. In other words, if a(m) = a(n), and min(m, n) > 1, then m = n. - Muhammed Hedayet, May 25 2016 Cosgrave & Dilcher propose the name Gauss factorial. Indeed the sequence is the special case N = n of the Gauss factorial N_n! = Product_{1<=j<=N, gcd(j, n)=1} j (see A216919). - Peter Luschny, Feb 07 2018 REFERENCES G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, Fifth ed., Clarendon Press, Oxford, 2003, Theorem 129, p. 102. 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). LINKS T. D. Noe, Table of n, a(n) for n = 1..200 J. B. Cosgrave and K. Dilcher, Extensions of the Gauss-Wilson Theorem, Integers: Electronic Journal of Combinatorial Number Theory, 8 (2008) J. B. Cosgrave, K. Dilcher, The multiplicative orders of certain Gauss factorials, Intl. J. Number Theory 7 (1) (2011) 145-171. S. W. Golomb and William Small, Problem E1045, Amer. Math. Monthly, 60 (1953), 422. Muhammed H. Islam and Shahriar Manzoor, φ1 and phitorial are injections, for any positive integer N, where N > 1 Laszlo Toth, Weighted gcd-sum functions, J. Integer Sequences, 14 (2011), Article 11.7.7 Eric Weisstein's World of Mathematics, Wilson's Theorem FORMULA a(n) = n^phi(n)*Product_{d|n} (d!/d^d)^mu(n/d); phi=A000010 is the Euler totient function and mu=A008683 the Moebius function (Tom M. Apostol, Introduction to Analytic Number Theory, New York 1984, p. 48). - Franz Vrabec, Jul 08 2005 a(n) = n!/A066570(n). - R. J. Mathar, Mar 10 2011 A001221(a(n)) = A000720(n) - A001221(n) = A048865(n). A006530(a(n)) = A136548(n). - Enrique Pérez Herrero, Jul 23 2011 a(n) = A124441(n)*A124442(n). - M. F. Hasler, Jul 23 2011 a(n) == (-1)^A211487(n) (mod n). - Vladimir Shevelev, May 13 2012 MAPLE A001783 := proc(n) local i, t1; t1 := 1; for i from 1 to n do if gcd(i, n)=1 then t1 := t1*i; fi; od; t1; end; A001783 := proc(n) local i; mul(i, i=select(k->igcd(n, k)=1, [\$1..n])) end; # Peter Luschny, Oct 30 2010 MATHEMATICA A001783[n_]:=Times@@Select[Range[n], CoprimeQ[n, #]&]; Array[A001783, 20] (* Enrique Pérez Herrero, Jul 23 2011 *) PROG (PARI) A001783(n)=prod(k=2, n-1, k^(gcd(k, n)==1))  \\ M. F. Hasler, Jul 23 2011 (PARI) a(n)=my(f=factor(n), t=n^eulerphi(f)); fordiv(f, d, t*=(d!/d^d)^moebius(n/d)); t \\ Charles R Greathouse IV, Nov 05 2015 (Haskell) a001783 = product . a038566_row -- Reinhard Zumkeller, Mar 04 2012, Aug 26 2011 (Sage) def Gauss_factorial(N, n): return mul(j for j in (1..N) if gcd(j, n) == 1) def A001783(n): return Gauss_factorial(n, n) [A001783(n) for n in (1..25)] # Peter Luschny, Oct 01 2012 CROSSREFS Cf. A023896, A066570, A038566, A124441, A216919. Sequence in context: A115031 A030418 A037277 * A095996 A308943 A061098 Adjacent sequences:  A001780 A001781 A001782 * A001784 A001785 A001786 KEYWORD nonn,nice,easy AUTHOR EXTENSIONS More terms from James A. Sellers, Dec 23 1999 STATUS approved

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Last modified October 17 06:08 EDT 2019. Contains 328106 sequences. (Running on oeis4.)