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A084057 a(n) = 2*a(n-1)+4*a(n-2), a(0)=1, a(1)=1. 36
1, 1, 6, 16, 56, 176, 576, 1856, 6016, 19456, 62976, 203776, 659456, 2134016, 6905856, 22347776, 72318976, 234029056, 757334016, 2450784256, 7930904576, 25664946176, 83053510656, 268766806016, 869747654656, 2814562533376, 9108115685376, 29474481504256 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Inverse binomial transform of A001077. Binomial transform of expansion of cosh(sqrt(5)x) (1,0,5,0,25,...).

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 5 times the bottom to get the new top. The limit of the sequence of fractions is sqrt(5). - Cino Hilliard, Sep 25 2005

From Mark Dols, Jul 24 2009 (Start):

Numerators of fractions in the approximation of the square root of 5 satisfying: a(n)= (a(n-1)+ c)/(a(n-1)+1); with c=5 and a(1)=1. For denominators see A063727.

(End)

Equals right border of triangle A143969. (1, 6, 16, 56,...) = row sums of triangle A143969 and INVERT transform of (1, 5, 5, 5,...). - Gary W. Adamson, Sep 06 2008

a(n) is the number of compositions of n when there are 1 type of 1 and 5 types of other natural numbers. - Milan Janjic, Aug 13 2010

From Gary W. Adamson, Jul 30 2016 (Start):

The sequence is case N=1 in an infinite set obtained by taking powers of the 2x2 matrix M = [(1,5); (1,N)], then extracting the upper left terms. The infinite set begins:

N=1 (A084057):  1, 6, 16,  56,  176,   576,   1856,...

N=2 (A108306):  1, 6, 21,  81,  306,  1161,   4401,...

N=3 (A164549):  1, 6, 26, 116,  516,  2296,  10216,...

N=4 (A015449):  1, 6, 31, 161,  836,  4341,  22541,...

N=5 (A000400):  1, 6, 36, 216, 1296,  7776,  46656,...

N=6 (A049685):  1, 6, 41, 281, 1926, 13201,  90481,...

N=7 (.......):  1, 6, 46, 356, 2756, 21336, 222712,...

...Sequences in the above set can be obtained by taking INVERT transforms of the following:

N=1 INVERT transform of (1, 5,  5,  5,   5,   5,...

N=2 ..."......"......". (1, 5, 10, 20,  40,   80,...

N=3 ..."......"......". (1, 5, 15, 45, 135,  405,...

N=4 ..."......"......". (1, 5, 20, 80, 320, 1280,...

...with the pattern.... (1, 5, N*5, (N^2)*5, (N^3)*5,...

It appears that a sequence generated from powers (n>0) of the matrix  P = [(1,a); (1,b)], (a,b,>0) then extracting the upper left terms; is equal to the INVERT transform of the sequence starting: (1, a, b*a, (b^2)*a, (b^3)*a,...). (End)

REFERENCES

John Derbyshire, Prime Obsession, Joseph Henry Press, April 2004, see p. 16.

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..1000

Index entries for linear recurrences with constant coefficients, signature (2,4).

FORMULA

a(n) = ((1+sqrt(5))^n + (1-sqrt(5))^n)/2.

G.f.: (1-x) / (1-2*x-4*x^2).

E.g.f.: exp(x) * cosh(sqrt(5)*x).

a(2n+1) = 2*a(n)*a(n+1)-(-4)^n. - Mario Catalani (mario.catalani(AT)unito.it), Jun 13 2003

a(n) = sum{k=0..floor(n/2), binomial(n, 2k)5^k }. - Paul Barry, Jul 25 2004

a(n) = Sum_{k, 0<=k<=n} A098158(n,k)*5^(n-k). - Philippe Deléham, Dec 26 2007

a(n) = 2^(n-1)*A000032(n). - Mark Dols, Jul 24 2009

If p[1]=1, and p[i]=5, (i>1), and if A is Hessenberg matrix of order n defined by: A[i,j]=p[j-i+1], (i<=j), A[i,j]:=-1, (i=j+1), and A[i,j]:=0 otherwise. Then, for n>=1, a(n)=det A. - Milan Janjic, Apr 29 2010

G.f.: G(0)/2, where G(k)= 1 + 1/(1 - x*(5*k-1)/(x*(5*k+4) - 1/G(k+1))); (continued fraction). - Sergei N. Gladkovskii, May 26 2013

MATHEMATICA

f[n_] := Simplify[((1 + Sqrt[5])^n + (1 - Sqrt[5])^n)/2]; Array[f, 28, 0] (* Or *)

LinearRecurrence[{2, 4}, {1, 1}, 28] (* Robert G. Wilson v, Sep 18 2013 *)

RecurrenceTable[{a[1] == 1, a[2] == 1, a[n] == 2 a[n-1] + 4 a[n-2]}, a, {n, 30}] (* Vincenzo Librandi, Jul 31 2016 *)

Table[2^(n-1) LucasL[n], {n, 0, 20}] (* Vladimir Reshetnikov, Sep 19 2016 *)

PROG

(Sage) from sage.combinat.sloane_functions import recur_gen2b; it = recur_gen2b(1, 1, 2, 4, lambda n: 0); [it.next() for i in xrange(1, 26)] # Zerinvary Lajos, Jul 09 2008

(Sage) [lucas_number2(n, 2, -4)/2 for n in xrange(0, 26)] # Zerinvary Lajos, Apr 30 2009

(PARI) lucas(n)=fibonacci(n-1)+fibonacci(n+1)

a(n)=lucas(n)/2*2^n \\ Charles R Greathouse IV, Sep 18 2013

(MAGMA) I:=[1, 1]; [n le 2 select I[n] else 2*Self(n-1)+4*Self(n-2): n in [1..30]]; // Vincenzo Librandi, Jul 31 2016

CROSSREFS

Cf. A046717, A002533, A143969.

a(n) = A087131(n)/2.

The following sequences (and others) belong to the same family: A001333, A000129, A026150, A002605, A046717, A015518, A084057, A063727, A002533, A002532, A083098, A083099, A083100, A015519.

Cf. A108306, A164549, A015449, A000400, A049685

Sequence in context: A192000 A032282 * A163302 A223028 A091649 A125628

Adjacent sequences:  A084054 A084055 A084056 * A084058 A084059 A084060

KEYWORD

easy,nonn

AUTHOR

Paul Barry, May 10 2003

STATUS

approved

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Last modified October 20 17:53 EDT 2018. Contains 316397 sequences. (Running on oeis4.)