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Narayana Cow Sequence


The Narayana cow sequence, named after the 14th-century Indian mathematician Narayana Pandita (Lin 2021), counts the cows in a herd that begins with one cow in the first year and in which each cow gives birth to one calf a year from the age of three onwards. The sequence for years n=1, 2, ... then gives 1, 1, 1 + 1 cow born = 2, 2 + 1 cow born = 3, 3 + 1 cow born = 4, 4 + 2 cows born = 6, and so on. Continuing gives the sequence 1, 1, 2, 3, 4, 6, 9, 13, 19, 28, 41, 60, ... (OEIS A000930).

The nth Narayana cow number N_n gives the number of compositions of a positive integer n involving only 1 and 3, the first few of which are shown in the following table.

nN_ncompositions of n using 1 and 3
111
211+1
323, 1+1+1
431+3, 3+1, 1+1+1+1
541+1+3, 1+3+1, 3+1+1, 1+1+1+1+1
663+3, 1+1+1+3, 1+1+3+1, 1+3+1+1, 3+1+1+1, 1+1+1+1+1+1

Based on its definition, the Narayana cow sequence is evidently related to both the Fibonacci sequence and Padovan sequence.

The terms of the sequence are given by the linear recurrence equation

 N_n=N_(n-1)+N_(n-3)
(1)

for n>=4, with N_1=1, N_2=1, and N_3=2. They have generating function

 f(x)=sum_(n=1)^inftyN_nx^(n-1)=-(x^2+1)/(x^3+x-1).
(2)

They also have a closed form in terms of a generalized hypergeometric function

 N_n=_3F_2((1-n)/3,(2-n)/3,-n/3;(1-n)/2,-n/2;-(27)/4).
(3)

The limit of ratios of consecutive terms is the supergolden ratio

 lim_(n->infty)(N_n)/(N_(n-1))=psi.
(4)

See also

Narayana Number, Narayana Triangle, Supergolden Ratio

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References

Lin, X. "On the Recurrence Properties of Narayana's Cows Sequence." Symmetry 13, 149, 2021. https://doi.org/10.3390/sym13010149.Narayana Pandita. Ganita Kaumudi. 1356.Pegg, E. Jr. "Shattering the Plane with Twelve New Substitution Tilings Using 2, phi, psi, chi, rho." Mar. 7, 2019. https://blog.wolfram.com/2019/03/07/shattering-the-plane-with-twelve-new-substitution-tilings-using-2-phi-psi-chi-rho/.Sloane, N. J. A. Sequence A000930 in "The On-Line Encyclopedia of Integer Sequences."

Referenced on Wolfram|Alpha

Narayana Cow Sequence

Cite this as:

Weisstein, Eric W. "Narayana Cow Sequence." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/NarayanaCowSequence.html

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