login
Expansion of x*(1 + 2*x)/((1 - x)*(1 + x)*(1 - x - x^2)).
0

%I #10 May 19 2023 08:03:30

%S 0,1,3,5,10,16,28,45,75,121,198,320,520,841,1363,2205,3570,5776,9348,

%T 15125,24475,39601,64078,103680,167760,271441,439203,710645,1149850,

%U 1860496,3010348,4870845,7881195,12752041,20633238,33385280,54018520,87403801,141422323,228826125,370248450

%N Expansion of x*(1 + 2*x)/((1 - x)*(1 + x)*(1 - x - x^2)).

%C Apparently (for n > 0), numbers that have a unique partition into a sum of distinct Lucas numbers (A000204).

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/LucasNumber.html">Lucas Number</a>

%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,-1,-1).

%F G.f.: x*(1 + 2*x)/((1 - x)*(1 + x)*(1 - x - x^2)).

%F a(n) = a(n-1) + 2*a(n-2) - a(n-3) - a(n-4).

%F a(n) = Lucas(n+1) - (3 - (-1)^n)/2.

%F a(n) = floor(phi^(n+1)) - 1, where phi = (1 + sqrt(5))/2 is the golden ratio (A001622).

%F a(n) = Sum_{k>=0} A051601(n-k,k) (conjectured). - _Greg Dresden_, May 18 2023

%t CoefficientList[Series[x (1 + 2 x)/((1 - x) (1 + x) (1 - x - x^2)) , {x, 0, 40}], x]

%t LinearRecurrence[{1, 2, -1, -1}, {0, 1, 3, 5}, 41]

%t Table[LucasL[n + 1] - (3 - (-1)^n)/2, {n, 0, 40}]

%t Table[Floor[GoldenRatio^(n + 1)] - 1, {n, 0, 40}]

%o (PARI) a(n) = fibonacci(n) + fibonacci(n+2) + ((-1)^n - 3)/2; \\ _Altug Alkan_, Mar 25 2018

%Y Cf. A000071, A000204, A001622, A003263, A014217, A054770, A294203.

%K nonn,easy

%O 0,3

%A _Ilya Gutkovskiy_, Mar 25 2018