

A002532


a(n) = 2*a(n1) + 5*a(n2), a(0) = 0, a(1) = 1.
(Formerly M1923 N0758)


32



0, 1, 2, 9, 28, 101, 342, 1189, 4088, 14121, 48682, 167969, 579348, 1998541, 6893822, 23780349, 82029808, 282961361, 976071762, 3366950329, 11614259468, 40063270581, 138197838502, 476712029909, 1644413252328, 5672386654201
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OFFSET

0,3


COMMENTS

The same sequence may be obtained by the following process. Starting a priori with the fraction 1/1, the numerators of fractions built according to the rule: add top and bottom to get the new bottom, add top and 6 times the bottom to get the new top. The limit of the sequence of fractions is sqrt(6).  Cino Hilliard, Sep 25 2005
For n>=2, number of ordered partitions of n1 into parts of sizes 1 and 2 where there are two types of 1 (singletons) and five types of 2 (twins). For example, the number of possible configurations of families of n1 male (M) and female (F) offspring considering only single births and twins, where the birth order of M/F/pairoftwins is considered and there are five types of twins; namely, both F (identical twins), both F (fraternal twins), both M (identical), both M (fraternal), or one F and one M  where birth order within a pair of twins itself is disregarded. In particular, for a(3)=9, two children could be either: (1) F, then M; (2) M, then F; (3) F,F; (4) M,M; (5) F,F identical twins; (6) F,F fraternal twins; (7) M,M identical twins; (8) M,M fraternal twins; or (9) M,F twins (emphasizing that birth order is irrelevant here when children are the same gender, when two children are within the same pair of twins and when pairs of twins have both the same gender(s) and identicalvsfraternal characteristics).  Rick L. Shepherd, Sep 19 2004
Pisano period lengths: 1, 2, 3, 4, 4, 6, 24, 8, 3, 4, 120, 12, 56, 24, 12, 16, 288, 6, 18, 4, ... .  R. J. Mathar, Aug 10 2012


REFERENCES

John Derbyshire, Prime Obsession, Joseph Henry Press, April 2004, see p. 16.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
A. Tarn, Approximations to certain square roots and the series of numbers connected therewith, Mathematical Questions and Solutions from the Educational Times, 1 (1916), 812.


LINKS



FORMULA

From Mario Catalani (mario.catalani(AT)unito.it), Jun 14 2003: (Start)
a(2*n+1) = 5*a(n)^2 + a(n+1)^2.
G.f.: x/(12*x5*x^2).
E.g.f.: exp(x)*sinh(sqrt(6)*x)/sqrt(6).
a(n) = ((1+sqrt(6))^n  (1sqrt(6))^n)/(2*sqrt(6)). (end)
a(n) = Sum_{k=0..n} binomial(n, 2*k+1)*6^k.  Paul Barry, Sep 29 2004
G.f.: G(0)*x/(2*(1x)), where G(k)= 1 + 1/(1  x*(6*k1)/(x*(6*k+5)  1/G(k+1))); (continued fraction).  Sergei N. Gladkovskii, May 26 2013


EXAMPLE

G.f. = x + 2*x^2 + 9*x^3 + 28*x^4 + 101*x^5 + 342*x^6 + 1189*x^7 + ...


MAPLE



MATHEMATICA

Expand[Table[((1 + Sqrt[6])^n  (1  Sqrt[6])^n)/(2Sqrt[6]), {n, 0, 25}]] (* Zerinvary Lajos, Mar 22 2007 *)
a[n_]:=(MatrixPower[{{1, 2}, {1, 3}}, n].{{1}, {1}})[[2, 1]]; Table[Abs[a[n]], {n, 1, 40}] (* Vladimir Joseph Stephan Orlovsky, Feb 19 2010 *)
LinearRecurrence[{2, 5}, {0, 1}, 30] (* Harvey P. Dale, Nov 03 2011 *)


PROG

(Sage) from sage.combinat.sloane_functions import recur_gen2; it = recur_gen2(0, 1, 2, 5); [next(it) for i in range(30)] # Zerinvary Lajos, Jun 25 2008
(Sage) [lucas_number1(n, 2, 5) for n in range(0, 26)] # Zerinvary Lajos, Apr 22 2009
(Magma) [Floor(((1+Sqrt(6))^n(1Sqrt(6))^n)/(2*Sqrt(6))): n in [0..30]]; // Vincenzo Librandi, Aug 15 2011
(Magma) [n le 2 select n1 else 2*Self(n1) + 5*Self(n2): n in [1..30]]; // G. C. Greubel, Jan 08 2018


CROSSREFS

Cf. A015581 (similar application, but no distinguishing identical vs. fraternal twins).
The following sequences (and others) belong to the same family: A001333, A000129, A026150, A002605, A046717, A015518, A084057, A063727, A002533, A002532, A083098, A083099, A083100, A015519.


KEYWORD

nonn,easy


AUTHOR



EXTENSIONS



STATUS

approved



