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A002001
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a(n) = 3*4^(n-1), n>0; a(0)=1.
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45
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1, 3, 12, 48, 192, 768, 3072, 12288, 49152, 196608, 786432, 3145728, 12582912, 50331648, 201326592, 805306368, 3221225472, 12884901888, 51539607552, 206158430208, 824633720832, 3298534883328, 13194139533312, 52776558133248, 211106232532992, 844424930131968
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OFFSET
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0,2
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COMMENTS
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Second binomial transform of (1,1,4,4,16,16,...) = (3*2^n+(-2)^n)/4. - Paul Barry, Jul 16 2003
Number of vertices (or sides) formed after the (n-1)-th iterate towards building a Koch's snowflake. - Lekraj Beedassy, Jan 24 2005
For n>=1, a(n) is equal to the number of functions f:{1,2...,n}->{1,2,3,4} such that for a fixed x in {1,2,...,n} and a fixed y in {1,2,3,4} we have f(x)<>y. - Aleksandar M. Janjic and Milan Janjic, Mar 27 2007
a(n) = (n+1) terms in the sequence (1, 2, 3, 3, 3,...) dot (n+1) terms in the sequence (1, 1, 3, 12, 48,...). Example: a(4) = 192 = (1, 2, 3, 3, 3) dot (1, 1, 3, 12, 48) = (1 + 2 + 9 + 36 + 144). - Gary W. Adamson, Aug 03 2010
a(n) is the number of compositions of n when there are 3 types of each natural number. - Milan Janjic, Aug 13 2010
See A178789 for the number of acute (= exterior) angles of the Koch snowflake referred to in the above comment by L. Beedassy. - M. F. Hasler, Dec 17 2013
After 1, subsequence of A033428. - Vincenzo Librandi, May 26 2014
a(n) counts walks (closed) on the graph G(1-vertex;1-loop x3,2-loop x3,3-loop x3,4-loop x3,...). - David Neil McGrath, Jan 01 2015
For n > 1, a(n) are numbers k such that (2^(k-1) mod k)/(2^k mod k) = 2; 2^(a(n)-1) == 2^(2n-1) (mod a(n)) and 2^a(n) == 2^(2n-2) (mod a(n)). - Thomas Ordowski, Apr 22 2020
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..300
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 456
P. Kernan, Koch Snowflake [Broken link]
C. Lanius, The Koch Snowflake
Eric Weisstein's World of Mathematics, Koch Snowflake
Wikipedia, Koch snowflake
Index to divisibility sequences
Index entries for linear recurrences with constant coefficients, signature (4).
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FORMULA
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From Paul Barry, Apr 20 2003: (Start)
a(n) = (3*4^n + 0^n)/4 (with 0^0=1).
E.g.f.: (3*exp(4*x) + 1)/4. (End)
With interpolated zeros, this has e.g.f. (3*cosh(2*x) + 1)/4 and binomial transform A006342. - Paul Barry, Sep 03 2003
a(n) = Sum_{j=0..1} Sum_{k=0..n} C(2n+j, 2k). - Paul Barry, Nov 29 2003
G.f.: (1-x)/(1-4*x). The sequence 1, 3, -12, 48, -192... has g.f. (1+7*x)/(1+4*x). - Paul Barry, Feb 12 2004
a(n) = 3*Sum_(k=0..(n-1)} a(k). - Adi Dani, Jun 24 2011
G.f.: 1/(1-3*Sum_{k>=1} x^k). - Joerg Arndt, Jun 24 2011
Row sums of triangle A134316. - Gary W. Adamson, Oct 19 2007
a(n) = A011782(n) * A003945(n). - R. J. Mathar, Jul 08 2009
If p[1]=3, p[i]=3, (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
a(n) = 4*a(n-1), a(0)=1, a(1)=3. - Vincenzo Librandi, Dec 31 2010
G.f.: 1 - G(0) where G(k) = 1 - 1/(1-3*x)/(1-x/(x-1/G(k+1) )); (recursively defined continued fraction). - Sergei N. Gladkovskii, Jan 25 2013
G.f.: x+2*x/(G(0)-2), where G(k) = 1 + 1/(1 - x*(3*k+1)/(x*(3*k+4) + 1/G(k+1))); (continued fraction). - Sergei N. Gladkovskii, May 26 2013
a(n) = ceiling(3*4^(n-1)). - Wesley Ivan Hurt, Dec 17 2013
Construct the power matrix T(n,j)=[A(n)^*j]*[S(n)^*(j-1)] where A(n)=(3,3,3...) and S(n)=(0,1,0,0...). (* is convolution operation). Then T(n,j) counts n-walks containing j loops on the single vertex graph above and a(n) = Sum_{j=1...n} T(n,j). (S(n)^*0=I.) - David Neil McGrath, Jan 01 2015
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MAPLE
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A002001:=n->ceil(3*4^(n-1)); seq(A002001(n), n=0..30); # Wesley Ivan Hurt, Dec 17 2013
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MATHEMATICA
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Table[Ceiling[3*4^(n - 1)], {n, 0, 30}] (* Wesley Ivan Hurt, May 26 2014 *)
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PROG
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(MAGMA) [ (3*4^n+0^n)/4: n in [0..22] ]; // Klaus Brockhaus, Aug 15 2009
(PARI) v=vector(100, n, 3*4^(n-2)); v[1]=1; v \\ Charles R Greathouse IV, May 19, 2011
(PARI) A002001=n->if(n, 3*4^(n-1), 1) \\ M. F. Hasler, Dec 17 2013
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CROSSREFS
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First difference of 4^n (A000302).
Cf. A134316.
Sequence in context: A254942 A077828 A164346 * A113956 A323261 A103943
Adjacent sequences: A001998 A001999 A002000 * A002002 A002003 A002004
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KEYWORD
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nonn,easy
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AUTHOR
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N. J. A. Sloane, Dec 11 1996
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STATUS
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approved
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