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A249629
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Number of strings of length n over a 4-letter alphabet that begin with a nontrivial palindrome.
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10
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0, 0, 4, 28, 124, 532, 2164, 8788, 35284, 141628, 567004, 2269948, 9081724, 36334492, 145345564, 581412508, 2325680284, 9302841652, 37211487124, 148846430068, 595386201844, 2381546731732, 9526188851284, 38104763100628, 152419060098004, 609676271166388, 2438705115439924, 9754820584849588
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OFFSET
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0,3
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COMMENTS
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A nontrivial palindrome is a palindrome of length 2 or greater. (I.e., "1" is a trivial palindrome, but "11" and "121" are nontrivial palindromes.)
For example, 0032 is a is a string of length 4 over a 4-letter alphabet that begins with a nontrivial palindrome (00).
4 divides a(n) for all n.
Number of walks of n steps that begin with a palindromic sequence on the complete graph K_4 with loops. (E.g., 0, 1, 1, 0, 3, 1, 2 is a valid walk with 7 steps and begins with the palindromic sequence '0110'.)
lim n -> infinity a(n)/4^n ~ 0.5415013252744246 is the probability that a random, infinite base-4 string begins with a nontrivial palindrome.
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LINKS
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FORMULA
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a(0) = 0; a(1) = 0; a(n+1) = 4*a(n) + 4^ceiling((n+1)/2) - a(ceiling((n+1)/2)).
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EXAMPLE
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for n=3 the a(3) = 28 solutions are: 000, 001, 002, 003, 010, 020, 030, 101, 110, 111, 112, 113, 121, 131, 202, 212, 220, 221, 222, 223, 232, 303, 313, 323, 330, 331, 332, 333.
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MATHEMATICA
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a249629[n_] := Block[{f},
f[0] = f[1] = 0;
f[x_] := 4*f[x - 1] + 4^Ceiling[x/2] - f[Ceiling[x/2]];
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PROG
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(Ruby) seq = [0, 0]; (2..N).each{ |i| seq << 4 * seq[i-1] + 4**((i+1)/2) - seq[(i+1)/2] }
(Haskell)
import Data.Ratio
a 0 = 0; a 1 = 0;
a n = 4 * a(n - 1) + 4^ceiling(n % 2) - a(ceiling(n % 2)) -- Peter Kagey, Aug 13 2015
(Magma) [0] cat [n le 1 select 0 else 4*Self(n-1) + 4^Ceiling((n)/2) - Self(Ceiling((n)/2)): n in [1..40]]; // Vincenzo Librandi, Aug 20 2015
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CROSSREFS
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KEYWORD
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easy,nonn,walk
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AUTHOR
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STATUS
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approved
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