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A281254
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Concatenate the decimal numbers n n-2 n-4 ...5 3 1 2 4 ... n-5 n-3 n-1 if n is odd, or n n-2 n-4 ... 6 4 2 1 3 5 ... n-5 n-3 n-1 if n is even.
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3
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1, 21, 312, 4213, 53124, 642135, 7531246, 86421357, 975312468, 10864213579, 1197531246810, 121086421357911, 13119753124681012, 1412108642135791113, 151311975312468101214, 16141210864213579111315, 1715131197531246810121416
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,2
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COMMENTS
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The old name was "Alternately concatenate the decimal digits from back to front 1...n such that n is always to the left.".
a(40714), with 192464 decimal digits, is the sequence's first (probable) prime. - Hans Havermann, Dec 17 2019
The terms at even indices are the same as the even-indexed terms of A053063. - Hans Havermann, Jan 16 2020
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LINKS
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EXAMPLE
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a(13) = 13119753124681012.
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MAPLE
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b:= proc(n) b(n):= `if`(n=1, 1, parse(cat(a(n-1), n))) end:
a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(n, b(n-1)))) end:
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MATHEMATICA
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f[n_] := Fold[ If[ Mod[n +#2, 2] == 0, #2, #1]*10^IntegerLength@ If[ Mod[n +#2, 2] == 0, #1, #2] +If[ Mod[n +#2, 2] == 1, #2, #1] &, 0, Range@ n]; Array[f, 17]
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PROG
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(Python)
def a(n):
....if n==1:
........return ["1"]
....return [str(n)]+a(n-1)[::-1]
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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