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A110523 Expansion of (1 + x)/(1 + x + 3*x^2). 5
1, 0, -3, 3, 6, -15, -3, 48, -39, -105, 222, 93, -759, 480, 1797, -3237, -2154, 11865, -5403, -30192, 46401, 44175, -183378, 50853, 499281, -651840, -846003, 2801523, -263514, -8141055, 8931597, 15491568, -42286359, -4188345, 131047422, -118482387, -274659879, 630107040, 193872597 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Row sums of number triangle A110522.

The sequence a(n) is conjugate with A214733 since the following alternative relations: either ((-1 + i*sqrt(11))/2)^n = a(n) + A214733(n)*(-1 + i*sqrt(11))/2 or ((-1 - i*sqrt(11))/2)^n = a(n) + A214733(n)*(-1 - i*sqrt(11))/2. We have a(n+1) = -3*A214733(n), A214733(n+1) = a(n) - A214733(n). We note that sequences A110512 and A001607 are conjugated in a similar way. The above relations are connected with the Gauss sums, for example if e := exp(i*2Pi/11) then e + e^3 + e^4 + e^5 + e^9 = (-1 + i*sqrt(11))/2, and e^2 + e^6 + e^7 + e^8 + e^10 = (-1 - i*sqrt(11))/2, which is equivalent to the system of sums: Sum_{k=1..5} cos(2Pi*k/11) = -1/2 and Sum_{k=1..5} sin(2Pi*k/11) = sqrt(11)/2, and which is equivalent to the system of products: P_{k=1..5} cos(2Pi*k/11) = -1/32 and P_{k=1..5} sin(2Pi*k/11) = sqrt(11)/32 - for details see Witula's book. At last we note that ((-1 + i*sqrt(11))/2)^n + ((-1 - i*sqrt(11))/2)^n = 2*a(n) - A214733(n). - Roman Witula, Jul 27 2012

REFERENCES

R. Witula, On Some Applications of Formulae for Unimodular Complex Numbers, Jacek Skalmierski's Press, Gliwice 2011.

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000

Index entries for linear recurrences with constant coefficients, signature (-1,-3).

FORMULA

a(n) = Sum_{k=0..n} Sum_{j=0..n} (-1)^(n-j)*C(n, j)*(-3)^(j-k)*C(k, j-k).

From Roman Witula, Jul 27 2012: (Start)

a(n+2) + a(n+1) + 3*a(n) = 0.

a(n+1) = -(3*i*sqrt(11)/11)*(((-1 - i*sqrt(11))/2)^n - ((-1 + i*sqrt(11))/2)^n). (End)

MATHEMATICA

CoefficientList[Series[(1 + x)/(1 + x + 3 x^2), {x, 0, 50}], x] (* G. C. Greubel, Aug 30 2017 *)

LinearRecurrence[{-1, -3}, {1, 0}, 40] (* Harvey P. Dale, Jul 02 2022 *)

PROG

(PARI) x='x+O('x^50); Vec((1 + x)/(1 + x + 3 x^2)) \\ G. C. Greubel, Aug 30 2017

CROSSREFS

Cf. A214733.

Sequence in context: A318540 A325879 A325865 * A145597 A143418 A336452

Adjacent sequences:  A110520 A110521 A110522 * A110524 A110525 A110526

KEYWORD

easy,sign

AUTHOR

Paul Barry, Jul 24 2005

STATUS

approved

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Last modified August 13 14:01 EDT 2022. Contains 356091 sequences. (Running on oeis4.)