This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A174663 a(n) is the number of solutions to the congruence Sum_{k=1..n} x_k == 1 (mod 2n), where x_k are distinct elements of the set {0, 1, ..., 2n}, k = 1..n. 1
 1, 4, 18, 192, 3000, 56160, 1234800, 32256000, 979776000, 33566400000, 1279932192000, 53950908211200, 2490951541478400, 124914111972249600, 6761428395321600000, 393000294670663680000, 24412776290272161792000, 1613964246117021646848000, 113146793781167491817472000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 REFERENCES V. S. Shevelev, On number of solutions of congruence Sum{i=1,...,s}x_i==r(modk), Izvestia Vuzov of the North-Caucasus region, Nature sciences, 2 (1997), 25-37 (in Russian). LINKS FORMULA a(n) = ((n-1)!/2)*Sum_{d|n} ( -1)^(n+d)*mu(n/d)*C(2d,d), where mu(n) is the Mobius function A008683. EXAMPLE If n=2, then we have the congruence x_1 + x_2 == 1 (mod 4), x_i is in {0,1,2,3}. Here we have 4 solutions: (0,1), (1,0), (2,3), (3,2); therefore a(2)=4. PROG (PARI) a(n) = ((n-1)!/2) * sumdiv(n, d, ( -1)^(n+d) * moebius(n/d) * binomial(2*d, d) ); vector(33, n, a(n)) \\ Joerg Arndt, Sep 05 2018 CROSSREFS Cf. A008683, A000984. Sequence in context: A260970 A154731 A201346 * A197786 A242083 A327335 Adjacent sequences:  A174660 A174661 A174662 * A174664 A174665 A174666 KEYWORD nonn AUTHOR Vladimir Shevelev, Mar 26 2010, Apr 09 2010, Jun 29 2010 EXTENSIONS a(6) corrected and more terms from Joerg Arndt, Sep 05 2018 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 17 19:02 EDT 2019. Contains 327137 sequences. (Running on oeis4.)