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Sum of Fibonacci and tribonacci numbers: a(n) = A000073(n) + A000045(n).
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%I #28 Nov 29 2021 09:18:57

%S 0,1,2,3,5,9,15,26,45,78,136,238,418,737,1304,2315,4123,7365,13193,

%T 23694,42655,76958,139126,251974,457112,830501,1510930,2752175,

%U 5018581,9160293,16734631,30595694,55976389,102474674,187700488,343973242,630623826,1156594669

%N Sum of Fibonacci and tribonacci numbers: a(n) = A000073(n) + A000045(n).

%C In general, the sum of a second-order sequence with signature (a,b) and a third-order sequence with signature (x,y,z) will be a fifth-order sequence with signature (a+x,-x*a+b+y, -y*a+z-b*x,-a*z-b*y,-b*z). In this instance, a=b=x=y=z=1 resulting in a signature of (2,1,-1,-2,-1).

%H <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-1,-2,-1).

%F a(n) = A000073(n) + A000045(n).

%F a(n) = 2*a(n-1) + a(n-2) - a(n-3) - 2*a(n-4) - a(n-5) for n > 4 with a(0)=0, a(1)=1, a(2)=2, a(3)=3, a(4)=5.

%F G.f.: x*(1 - 2*x^2 - 2*x^3)/(1 - 2*x - x^2 + x^3 + 2*x^4 + x^5). - _Stefano Spezia_, Oct 15 2020

%t LinearRecurrence[{2, 1, -1, -2, -1}, {0, 1, 2, 3, 5}, 50] (* _Amiram Eldar_, Oct 15 2020 *)

%Y Cf. A000045, A000073.

%K nonn,easy

%O 0,3

%A _Gary Detlefs_, Oct 15 2020