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 A127876 Integers of the form (x^3)/6 + (x^2)/2 + x + 1. 7
 1, 13, 61, 172, 373, 691, 1153, 1786, 2617, 3673, 4981, 6568, 8461, 10687, 13273, 16246, 19633, 23461, 27757, 32548, 37861, 43723, 50161, 57202, 64873, 73201, 82213, 91936, 102397, 113623, 125641, 138478, 152161, 166717, 182173, 198556, 215893, 234211 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Generating polynomial is Schur's polynomial of degree 3. Schur's polynomials n degree are n-th first term of series expansion of e^x function. All polynomials are non-reducible and belonging to the An alternating Galois transitive group if n is divisible by 4 or to Sn symmetric Galois Group in other case (proof Schur, 1930). Let A be the Hessenberg matrix of order n, defined by: A[1,j]=1, A[i,i]:=3, (i>1), A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n>=4, a(n-2)=-coeff(charpoly(A,x),x^(n-3)). - Milan Janjic, Jan 27 2010 LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1). FORMULA From Colin Barker, May 15 2016: (Start) a(n) = (9*n^3-18*n^2+15*n-4)/2. a(n) = 4*a(n-1)-6*a(n-2)+4*a(n-3)-a(n-4) for n>4. G.f.: x*(1+2*x)*(1+7*x+x^2) / (1-x)^4. (End) E.g.f.: 2 + (9*x^3 + 9*x^2 + 6*x - 4)*exp(x)/2. - G. C. Greubel, Apr 29 2018 MATHEMATICA a = {}; Do[If[IntegerQ[1 + x + x^2/2 + x^3/6], AppendTo[a, 1 + x + x^2/2 + x^3/6]], {x, 1, 300}]; a Select[Table[x^3/6 + x^2/2 + x + 1, {x, 0, 200}], IntegerQ] (* Harvey P. Dale, Jan 06 2011 *) PROG (PARI) Vec(x*(1+2*x)*(1+7*x+x^2)/(1-x)^4 + O(x^50)) \\ Colin Barker, May 15 2016 (MAGMA) [(9*n^3-18*n^2+15*n-4)/2: n in [1..30]]; // G. C. Greubel, Apr 29 2018 (GAP) Filtered(List([0..150], x->(x^3)/6+(x^2)/2+x+1), IsInt); # Muniru A Asiru, Apr 30 2018 CROSSREFS Cf. A127873, A127874, A127875. Sequence in context: A081589 A270449 A139880 * A308461 A302560 A252970 Adjacent sequences:  A127873 A127874 A127875 * A127877 A127878 A127879 KEYWORD nonn,easy AUTHOR Artur Jasinski, Feb 04 2007 EXTENSIONS a(1) = 1 added by Harvey P. Dale, Jan 06 2011 STATUS approved

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Last modified June 13 05:21 EDT 2021. Contains 344981 sequences. (Running on oeis4.)