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A053063
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Pendulum numbers: alternately append n to beginning or end of previous term.
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3
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1, 21, 213, 4213, 42135, 642135, 6421357, 86421357, 864213579, 10864213579, 1086421357911, 121086421357911, 12108642135791113, 1412108642135791113, 141210864213579111315, 16141210864213579111315, 1614121086421357911131517, 181614121086421357911131517, 18161412108642135791113151719
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OFFSET
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1,2
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COMMENTS
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Eric Angelini and Kay Rosen suggest these could be called Pnuumlde numbers.
Hans Havermann finds that there is one more prime in the first 10000 terms, a(1399), which is a 4489-digit prime.
(End)
The third prime in the sequence is a(40714) with 192464 decimal digits. The terms at even indices are duplicated by the even-indexed terms of A281254, which is why this term is a prime for both sequences. - Hans Havermann, Jan 16 2020
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LINKS
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MATHEMATICA
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nxt[{n_, a_}]:={n+1, If[OddQ[n], (n+1)*10^IntegerLength[a]+a, a*10^IntegerLength[ n+1]+ n+1]}; NestList[nxt, {1, 1}, 30][[All, 2]] (* Harvey P. Dale, Jul 20 2022 *)
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PROG
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(PARI) lista(nn) = {print1(val=1, ", "); b = 0; for (n=2, nn, if (b, val = concat(Str(val), Str(n)), val = concat(Str(n), Str(val))); print1(val, ", "); b = 1 - b; ); } \\ Michel Marcus, Aug 11 2017
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CROSSREFS
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KEYWORD
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easy,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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