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2, 3, 21, 333, 9321, 404313, 25071021, 2103118293, 229256855121, 31493764788273, 5321869241361621, 1084772760899990253, 262436014353765070521, 74338674113237083780233, 24371443829213227706941821, 9155506650374192400724494213, 3906927095300068860534174415521
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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0,1
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LINKS
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FORMULA
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a(2n) == 21 (mod 100), n>0.
a(n) = (2n)! * [x^(2n)] sec(x) + sec(x)^2. - Ira M. Gessel, Apr 18 2013
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MAPLE
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A000111 := proc(n) if n = 0 then 1; else 2^n*abs(euler(n, 1/2)+euler(n, 1) ); end if; end proc:
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MATHEMATICA
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terms = 20;
CoefficientList[Sec[x] + Sec[x]^2 + O[x]^(2 terms - 1), x^2] Range[0, 2 terms - 2, 2]! (* Jean-François Alcover, Nov 19 2020 *)
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PROG
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(Python)
from itertools import accumulate, islice
def A141458_gen(): # generator of terms
yield 2
blist = (0, 1)
while True:
yield (blist := tuple(accumulate(reversed(blist), initial=0)))[-1] + (blist := tuple(accumulate(reversed(blist), initial=0)))[-1]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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