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 A261570 Concatenation of the palindromic numbers (A002113) in increasing order up to the n-th term and then in decreasing order. 3
 1, 121, 12321, 1234321, 123454321, 12345654321, 1234567654321, 123456787654321, 12345678987654321, 12345678911987654321, 123456789112211987654321, 1234567891122332211987654321, 12345678911223344332211987654321, 123456789112233445544332211987654321 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS By definition, all terms are palindromes. Inspired by A261493. There are no primes in this sequence up to a(1100). The least prime factors of a(n), n>=1, are: 1, 11, 3, 11, 41, 3, 239, 11, 3, 11, 11, 3, 11, 11, 3, 11, 11, 3, 71, 21557, 19, 17, 31, 181, 17, 353, 19, 31, 19, 29, 17, 29, 11616377, 214141, 19, 5471, 17, 13883, 3, 7, ..., . See A261411. The first (probable) prime in this sequence was found by David Broadhurst on Aug 25 2015: this is a(2007), a 21233-digit probable prime with central term 1008001. - N. J. A. Sloane, Aug 24 2015 LINKS M. F. Hasler, Table of n, a(n) for n = 1..108 EXAMPLE a(4) is the concatenation of 1, 2, 3 and 4, and then 3, 2 and 1 which results in 1234321. MATHEMATICA palQ[n_] := Reverse[idn = IntegerDigits@ n] == idn; s = Select[ Range @111, palQ]; f[n_] := FromDigits@ Flatten[ IntegerDigits@# & /@ Join[Take[s, n], Reverse@ Take[s, n - 1]]]; a = Array[f, 14] PROG (PARI) A002113(n)=if(n>9, (n-=9)*10+if(n>9, n\10, n), n)/* This "poor man's" version is valid only for n<109 */ A261570(n, S=A002113(n))={while(n--, S=Str(A002113(n), S, A002113(n))); eval(S)} \\ M. F. Hasler, Aug 29 2015 CROSSREFS Cf. A002113, A173426, A261493, A261411. Sequence in context: A321687 A002477 A173426 * A068117 A080162 A030174 Adjacent sequences: A261567 A261568 A261569 * A261571 A261572 A261573 KEYWORD nonn,easy,base AUTHOR Robert G. Wilson v, Aug 24 2015 STATUS approved

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Last modified September 27 20:41 EDT 2023. Contains 365714 sequences. (Running on oeis4.)