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 A075418 Sum of generalized tribonacci numbers A001644 and inverted tribonacci numbers A075298. 1
 4, 2, -2, 12, 12, 10, 54, 68, 108, 282, 422, 772, 1604, 2674, 5006, 9580, 16884, 31506, 58606, 105948, 196508, 362298, 662022, 1222772, 2249116, 4127210, 7605718, 13984148, 25701652, 47311458, 86994846, 159975004, 294336612, 541281698, 995529822, 1831291692, 3367998380, 6194717674 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS It seems that aside from a(2) the sequence is nonnegative. LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 FORMULA a(n) = a(n-2) + 4*a(n-3) + a(n-4) - a(n-6), a(0)=4, a(1)=2, a(2)=-2, a(3)=12, a(4)=12, a(5)=10. O.g.f.: (4 + 2*x - 6*x^2 - 6*x^3 + 2*x^4 + 4*x^5)/(1 - x^2 - 4*x^3 - x^4 + x^6). MATHEMATICA CoefficientList[Series[(4+2x-6x^2-6x^3+2x^4+4x^5)/(1-x^2-4x^3-x^4+x^6), {x, 0, 40}], x] PROG (PARI) my(x='x+O('x^40)); Vec((4+2*x-6*x^2-6*x^3+2*x^4+4*x^5)/(1-x^2 -4*x^3-x^4+x^6)) \\ G. C. Greubel, Apr 21 2019 (MAGMA) R:=PowerSeriesRing(Integers(), 40); Coefficients(R!( (4+2*x-6*x^2-6*x^3+2*x^4+4*x^5)/(1-x^2-4*x^3-x^4+x^6) )); // G. C. Greubel, Apr 21 2019 (Sage) ((4+2*x-6*x^2-6*x^3+2*x^4+4*x^5)/(1-x^2-4*x^3-x^4+x^6)).series(x, 40).coefficients(x, sparse=False) # G. C. Greubel, Apr 21 2019 CROSSREFS Cf. A001644, A075298. Sequence in context: A055630 A182700 A136202 * A199221 A096870 A261253 Adjacent sequences:  A075415 A075416 A075417 * A075419 A075420 A075421 KEYWORD easy,sign AUTHOR Mario Catalani (mario.catalani(AT)unito.it), Sep 14 2002 STATUS approved

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Last modified August 24 02:32 EDT 2019. Contains 326260 sequences. (Running on oeis4.)