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A204091
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The number of 1 X n Haunted Mirror Maze puzzles with a unique solution ending with a mirror.
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5
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1, 2, 10, 46, 210, 958, 4370, 19934, 90930, 414782, 1892050, 8630686, 39369330, 179585278, 819187730, 3736768094, 17045465010, 77753788862, 354678014290, 1617882493726, 7380056440050, 33664517212798, 153562473183890, 700483331493854, 3195291711101490
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OFFSET
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0,2
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COMMENTS
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The final slot can refer to a mirror of either type ('/' or '\'.)
A204092 counts the situation dropping the requirement that a board ends with a mirror. A204089 counts the situation where all mirrors have the same orientation. A204090 counts the boards with same orientation where the last square need not be a mirror.
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..1000
Samples of these types of puzzles can be found at this site.
Index entries for linear recurrences with constant coefficients, signature (5,-2).
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FORMULA
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a(n) = 5*a(n-1) - 2*a(n-2) with a(1) = 2, a(2) = 10.
a(n) = 2 * sum_{i=0}^{i=n-1} a(n)(2^(n-i)-1), a(0) = 1.
G.f.: (1-x)*(1-2*x) / (1-5*x+2*x^2).
a(n) = (2^(1-n) * ((5+sqrt(17))^n - (5-sqrt(17))^n))/sqrt(17) for n>0. - Colin Barker, Nov 26 2016
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EXAMPLE
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For n=2 we get the following ten boards:
('Z', '/')
('Z', '|')
('V', '/')
('V', '|')
('G', '/')
('G', '|')
('/', '/')
('/', '|')
('|', '/')
('|', '|')
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MATHEMATICA
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Join[{1}, LinearRecurrence[{5, -2}, {2, 10}, 40]]
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PROG
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(Python)
def a(n, d={0:1, 1:2, 2:10}):
if n in d:
return d[n]
d[n]=5*a(n-1) - 2*a(n-2)
return d[n]
(Python)
#Produces a(n) through enumeration and also displays boards::
def Lprint(n):
.print('The following generate boards with a unique solution')
.s=0
.for x in product(['Z', 'V', 'G', '/', '|'], repeat=n):
..if x[-1]=='/' or x[-1]=='|':
...#Splitting x up into a list pieces
...y=list(x)
...z=list()
...while y:
....if '/' in y or '|' in y:
.....if '/' in y and '|' in y:
......m = min(y.index('/'), y.index('|'))
.....else:
......if '/' in y:
.......m=y.index('/')
......else:
.......m=y.index('|')
.....if y[0] not in ['/', '|']: #Don't need to add blank pieces to z
......z.append(y[:m])
.....y=y[m+1:]
....else:
.....z.append(y)
.....y=[]
...#For each element in the list checking for Z&V together
...goodword=True
...for w in z:
....if 'Z' in w and 'V' in w:
.....goodword=False
...if goodword:
....s+=1
....print(x)
.return s
(PARI) Vec((1-x)*(1-2*x)/(1-5*x+2*x^2) + O(x^30)) \\ Colin Barker, Nov 26 2016
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CROSSREFS
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Cf. A204089-A204092.
Apart from signs and the initial term, same as A106709.
Sequence in context: A032389 A290923 A106709 * A221196 A137193 A006213
Adjacent sequences: A204088 A204089 A204090 * A204092 A204093 A204094
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KEYWORD
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nonn,easy
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AUTHOR
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David Nacin, Jan 10 2012
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STATUS
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approved
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