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A281253
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Alternately concatenate the decimal digits from front to back 1...n such that n is always to the right.
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2
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1, 12, 213, 3124, 42135, 531246, 6421357, 75312468, 864213579, 97531246810, 1086421357911, 119753124681012, 12108642135791113, 1311975312468101214, 141210864213579111315, 15131197531246810121416, 1614121086421357911131517
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listen;
history;
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OFFSET
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1,2
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COMMENTS
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a(n) is prime for n: 121, 1399 and no others < 3000.
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LINKS
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EXAMPLE
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a(13) = 12108642135791113.
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MAPLE
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b:= proc(n) b(n):= `if`(n=1, 1, parse(cat(n, a(n-1)))) end:
a:= proc(n) a(n):= `if`(n=1, 1, parse(cat(b(n-1), n))) end:
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MATHEMATICA
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f[n_] := Fold[ If[ Mod[n +#2, 2] == 0, #1, #2]*10^IntegerLength@ If[ Mod[n +#2, 2] == 0, #2, #1] +If[ Mod[n +#2, 2] == 0, #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 a(n-1)[::-1]+[str(n)]
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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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