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A025579 a(1)=1, a(2)=2, a(n) = 4*3^(n-3) for n >= 3. 5
1, 2, 4, 12, 36, 108, 324, 972, 2916, 8748, 26244, 78732, 236196, 708588, 2125764, 6377292, 19131876, 57395628, 172186884, 516560652, 1549681956, 4649045868, 13947137604, 41841412812, 125524238436, 376572715308, 1129718145924 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) is the sum of the numbers in row n+1 of the array defined in A025564 (and of the array in A024996).

a(n) is the number of (s(0), s(1), ..., s(n)) such that every s(i) is an integer, s(0) = 0, |s(i) - s(i-1)| = 1 for i = 1,2; |s(i) - s(i-1)| <= 1 for i >= 3.

Equals binomial transform of A095342: (1, 1, 5, 5, 17, 25, 61,...). - Gary W. Adamson, Mar 04 2010

LINKS

Colin Barker, Table of n, a(n) for n = 1..1000

Index entries for linear recurrences with constant coefficients, signature (3).

FORMULA

a(n) = A003946(n-2), n>2. - R. J. Mathar, May 28 2008

From Colin Barker, Oct 29 2019: (Start)

G.f.: x*(1 + x)*(1 - 2*x) / (1 - 3*x).

a(n) = 3*a(n-1) for n>3. (End)

MAPLE

seq( `if`(n<3, n, 4*3^(n-3)), n=1..30); # G. C. Greubel, Dec 26 2019

MATHEMATICA

Join[{1, 2}, 4*3^Range[0, 30]] (* or *) Join[{1, 2}, NestList[3#&, 4, 30]] (* Harvey P. Dale, Jun 27 2011 *)

PROG

(PARI) a(n)=max(n, 4*3^(n-3)) \\ Charles R Greathouse IV, Jun 28 2011

(PARI) Vec(x*(1+x)*(1-2*x)/(1-3*x) + O(x^30)) \\ Colin Barker, Oct 29 2019

(MAGMA) [1, 2] cat [4*3^(n-3): n in [3..30]]; // G. C. Greubel, Dec 26 2019

(Sage) [1, 2]+[4*3^(n-3) for n in (3..30)] # G. C. Greubel, Dec 26 2019

(GAP) Concatenation([1, 2], List([3..30], n-> 4*3^(n-3) )); # G. C. Greubel, Dec 26 2019

CROSSREFS

Cf. A003946, A024996, A025564, A095342.

Sequence in context: A149839 A149840 A224540 * A214936 A010552 A149841

Adjacent sequences:  A025576 A025577 A025578 * A025580 A025581 A025582

KEYWORD

nonn,easy

AUTHOR

Clark Kimberling

EXTENSIONS

Definition corrected by R. J. Mathar, May 28 2008

STATUS

approved

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Last modified September 26 16:43 EDT 2020. Contains 337374 sequences. (Running on oeis4.)