OFFSET
1,4
COMMENTS
a(n) is the length of the edge of the n-th equilateral triangle in the Padovan triangle spiral.
Partial sums of A000931. - Juri-Stepan Gerasimov, Jul 17 2009
Rising diagonal sums of triangle A152198. - John Molokach, Jul 09 2013
a(n) is the number of pairs of rabbits living at month n with the following rules: a pair of rabbits born in month n begins to procreate in month n + 2, procreates again in month n + 3, and dies at the end of this month (each pair therefore gives birth to 2 pairs); the first pair is born in month 1. - Robert FERREOL, Oct 16 2017
REFERENCES
Richter, Christian. "Tilings of convex polygons by equilateral triangles of many different sizes." Discrete Mathematics 343.3 (2020): 111745. (See Section 2.1.)
S. J. Tedford, Combinatorial identities for the Padovan numbers, Fib. Q., 57:4 (2019), 291-298.
LINKS
Muniru A Asiru, Table of n, a(n) for n = 1..1500
J.-L. Baril, Avoiding patterns in irreducible permutations, Discrete Mathematics and Theoretical Computer Science, Vol. 17, No. 3 (2016).
Eric Fernando Bravo, On concatenations of Padovan and Perrin numbers, Math. Commun. (2023) Vol 28, 105-119.
Alain Faisant, On the Padovan sequence, arXiv:1905.07702 [math.NT], 2019.
Ed Harris, Pete McPartlan and Brady Haran, The Plastic Ratio, Numberphile video (2019).
Mariana Nagy, Simon R. Cowell, and Valeriu Beiu, On the Construction of 3D Fibonacci Spirals, Mathematics (2024) Vol. 12, No. 2, 201.
Wikipedia, Padovan triangles
Index entries for linear recurrences with constant coefficients, signature (0,1,1).
FORMULA
a(n) = A000931(n+4).
G.f.: x * (1 + x) / (1 - x^2 - x^3) = x / (1 - x / (1 - x^2 / (1 + x / (1 - x / (1 + x))))). - Michael Somos, Jan 03 2013
a(1)=a(2)=a(3)=1, a(n) = a(n-2) + a(n-3) for n > 3. - Robert FERREOL, Oct 16 2017
EXAMPLE
a(6)=3 because 6+4=10 and A000931(10)=3.
G.f. = x + x^2 + x^3 + 2*x^4 + 2*x^5 + 3*x^6 + 4*x^7 + 5*x^8 + 7*x^9 + ... - Michael Somos, Jan 01 2019
MAPLE
a:=proc(n, p, q) option remember:
if n<=p then 1
elif n<=q then a(n-1, p, q)+a(n-p, p, q)
else add(a(n-k, p, q), k=p..q) fi end:
seq(a(n, 2, 3), n=0..100); # Robert FERREOL, Oct 16 2017
MATHEMATICA
Drop[ CoefficientList[ Series[(1 - x^2)/(1 - x^2 - x^3), {x, 0, 52}], x], 5] (* Or *) a[1] = a[2] = a[3] = 1; a[n_] := a[n] = a[n - 2] + a[n - 3]; Array[ a, 48] (* Robert G. Wilson v, Sep 30 2009 *)
a[ n_] := If[ n >= 0, SeriesCoefficient[ (x + x^2) / (1 - x^2 - x^3), {x, 0, n}], SeriesCoefficient[ (x + x^2) / (1 + x - x^3), {x, 0, Abs@n}]];
PROG
(GAP) a:=[1, 1, 1];; for n in [4..50] do a[n]:=a[n-2]+a[n-3]; od; a; # Muniru A Asiru, Aug 12 2018
(PARI) {a(n) = if( n>=0, polcoeff( (x + x^2) / (1 - x^2 - x^3) + x * O(x^n), n), polcoeff( (x + x^2) / (1 + x - x^3) + x * O(x^-n), -n))}; /* Michael Somos, Jan 01 2019 */
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Omar E. Pol, Nov 13 2007
EXTENSIONS
More terms from Robert G. Wilson v, Sep 30 2009
First comment clarified by Omar E. Pol, Aug 12 2018
STATUS
approved