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A279193
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Least positive integer whose decimal digits divide the plane into n regions (version for people who write 2 with a curlicue).
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1
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1, 2, 8, 28, 88, 288, 888, 2888, 8888, 28888, 88888, 288888, 888888, 2888888, 8888888, 28888888, 88888888, 288888888, 888888888, 2888888888, 8888888888, 28888888888, 88888888888, 288888888888, 888888888888, 2888888888888, 8888888888888, 28888888888888
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OFFSET
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1,2
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COMMENTS
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Equivalently, with offset 0, least positive integer with n holes in its decimal digits. Note that 2 written with a curlicue has one hole, 8 has two holes, 28 has three holes, etc., as in the illustration. See A249572 and A250256 for other versions.
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LINKS
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FORMULA
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a(n) = a(n-1) + 10*a(n-2) - 10*a(n-3) for n>4.
G.f.: x*(10*x^3 - 4*x^2 + x + 1)/((x - 1)*(10*x^2 - 1)). (End)
a(n) = ((26 - (13 - 4*sqrt(10))*(1 - (-1)^n))*sqrt(10^n) - 80)/90 for n>1, a(1)=1. - Bruno Berselli, Dec 15 2016
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MATHEMATICA
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Join[{1}, LinearRecurrence[{1, 10, -10}, {2, 8, 28}, 30]] (* Vincenzo Librandi, Dec 15 2016 *)
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PROG
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(Magma) I:=[1, 2, 8, 28]; [n le 4 select I[n] else Self(n-1)+10*Self(n-2)-10*Self(n-3): n in [1..30]]; // Vincenzo Librandi, Dec 15 2016
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CROSSREFS
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KEYWORD
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nonn,base,easy
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AUTHOR
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STATUS
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approved
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