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 A014601 Numbers congruent to 0 or 3 mod 4. 52
 0, 3, 4, 7, 8, 11, 12, 15, 16, 19, 20, 23, 24, 27, 28, 31, 32, 35, 36, 39, 40, 43, 44, 47, 48, 51, 52, 55, 56, 59, 60, 63, 64, 67, 68, 71, 72, 75, 76, 79, 80, 83, 84, 87, 88, 91, 92, 95, 96, 99, 100, 103, 104, 107, 108, 111, 112, 115, 116, 119, 120, 123, 124 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Discriminants of orders in imaginary quadratic fields (negated). [Comment corrected by Christopher E. Thompson, Dec 11 2016] n such that Langford-Skolem problem has a solution - see A014552. A014494(n) = A000217(a(n)); complement of A042963. - Reinhard Zumkeller, Oct 04 2004 Also called skew amenable numbers; a number n is skew amenable if there exist a set {a(i)} of integers satisfying the relations n = Sum_{i=1..n} a(i) = -Product_{i=1..n} a(i). Thus we have 8 = 1 + 1 + 1 + 1 + 1 + 1 - 2 + 4 = -(1*1*1*1*1*1*(-2)*4). - Lekraj Beedassy, Jan 07 2005 Possible nonpositive discriminants of quadratic equation a*x^2 + b*x + c or discriminants of binary quadratic forms a*x^2 + b*x*y + c*y^2. - Artur Jasinski, Apr 28 2008 Also, disregarding the 0 term, positive integers m such that, equivalently,   (i) +-1 +-2 +-... +-m is even for all choices of signs,   (ii) +-1 +-2 +-... +-m = 0 for some choices of signs,   (iii) for all -m <= k <= m, k = +-1 +-2 +-... +-(k-1) +-(k+1) +-(k+2) +-... +-m for at least one choice of signs. - Rick L. Shepherd, Oct 29 2008 A145768(a(n)) is even. - Reinhard Zumkeller, Jun 05 2012 Multiples of 4 interleaved with 1 less than multiples of 4. - Wesley Ivan Hurt, Nov 08 2013 ((2*k+0) + (2*k+1) + ... + (2*k+m-1) + (2*k+m)) is even if and only if m = a(n) for some n where k is any nonnegative integer. - Gionata Neri, Jul 24 2015 REFERENCES H. Cohen, Course in Computational Alg. No. Theory, Springer, 1993, pp. 514-5. A. Scholz and B. Schoeneberg, Einführung in die Zahlentheorie, 5. Aufl., de Gruyter, Berlin, New York, 1973, p. 108. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 S. F. Barger, Solution to problem 10454: Amenable Numbers, Amer. Math. Monthly Vol. 105 No. 4 April 1998 MAA Washington DC. S. R. Finch, Class number theory [Cached copy, with permission of the author] Heiko Harborth, Solution of Steinhaus's problem with plus and minus signs, Journal of Combinatorial Theory, Series A, Volume 12, Issue 2, March 1972, Pages 253-259. Rick L. Shepherd, Binary quadratic forms and genus theory, Master of Arts Thesis, University of North Carolina at Greensboro, 2013. Index entries for linear recurrences with constant coefficients, signature (1,1,-1). FORMULA a(n) = (n + 1)*2 + 1 - n mod 2. - Reinhard Zumkeller, Apr 21 2003 a(n) = Sum_{k=1..n} (2 - (-1)^k). - William A. Tedeschi, Mar 20 2008 A139131(a(n)) = A078636(a(n)). - Reinhard Zumkeller, Apr 10 2008 a(n) = a(n-1) + a(n-2) - a(n-3) for n > 2. G.f.: x*(3+x)/((1+x)*(x-1)^2). - R. J. Mathar, Sep 25 2009 a(n) = 2*n + (n mod 2). - Paolo Valzasina (p.valzasina(AT)gmail.com), Nov 24 2009 a(n) = (4*n - (-1)^n + 1)/2. - Bruno Berselli, Oct 06 2010 a(n) = 4*n - a(n-1) - 1 (with a(0) = 0). - Vincenzo Librandi, Dec 24 2010 a(n) = -A042948(-n) for all n in Z. - Michael Somos, Dec 27 2010 G.f.: 2*x / (1 - x)^2 + (1 / (1 - x) + 1 / (1 + x)) * x/2. - Michael Somos, Dec 27 2010 a(n) = Sum_{k>=0} A030308(n,k)*b(k) with b(0) = 3 and b(k) = 2^(k+1) for k > 0. - Philippe Deléham, Oct 17 2011 a(n) = ceiling((4/3)*ceiling(3*n/2)). - Clark Kimberling, Jul 04 2012 a(n) = 3n - 2*floor(n/2). - Wesley Ivan Hurt, Nov 08 2013 a(n) = A042948(n+1) - 1 for all n in Z. - Michael Somos, Jul 24 2015 a(n) + a(n+1) = A004767(n) for all n in Z. - Michael Somos, Jul 24 2015 EXAMPLE G.f. = 3*x + 4*x^2 + 7*x^3 + 8*x^4 + 11*x^5 + 12*x^6 + 15*x^7 + 16*x^8 + ... MAPLE A014601:=n->3*n-2*floor(n/2); seq(A014601(k), k=0..100); # Wesley Ivan Hurt, Nov 08 2013 MATHEMATICA aa = {}; Do[Do[Do[d = b^2 - 4 a c; If[d <= 0, AppendTo[aa, -d]], {a, 0, 50}], {b, 0, 50}], {c, 0, 50}]; Union[aa] (* Artur Jasinski, Apr 28 2008 *) Select[Range[0, 124], Or[Mod[#, 4] == 0, Mod[#, 4] == 3] &] (* Ant King, Nov 18 2010 *) CoefficientList[Series[2 x/(1 - x)^2 + (1/(1 - x) + 1/(1 + x)) x/2, {x, 0, 100}], x] (* Vincenzo Librandi, May 18 2014 *) a[ n_] := 2 n + Mod[n, 2]; (* Michael Somos, Jul 24 2015 *) PROG (MAGMA)[n: n in [0..200]|n mod 4 in {0, 3}]; // Vincenzo Librandi, Dec 24 2010 (PARI) {a(n) = 2*n + n%2}; /* Michael Somos, Dec 27 2010 */ (Haskell) a014601 n = a014601_list !! n a014601_list = [x | x <- [0..], mod x 4 `elem` [0, 3]] -- Reinhard Zumkeller, Jun 05 2012 CROSSREFS Cf. A004676, A042948, A079896. Cf. A274406. [Bruno Berselli, Jun 26 2016] Sequence in context: A288553 A295771 A285503 * A154708 A227148 A026444 Adjacent sequences:  A014598 A014599 A014600 * A014602 A014603 A014604 KEYWORD nonn,easy AUTHOR Eric Rains (rains(AT)caltech.edu) STATUS approved

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Last modified June 26 10:45 EDT 2019. Contains 324375 sequences. (Running on oeis4.)