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 A171775 a(n) = smallest number M such that there exist bases b_2, b_3, ..., b_n with the property that M written in base b_k is a k-digit palindrome for all k=2..n. 15
 1, 3, 5, 52, 130, 1885, 1073741824, 4398046511104 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) is no more than 2^[(n-1)*(n-2)] for n > 6 (and equals it for n = 7 and 8 at least).  The reason for this bound is that for this number for each length from n down to 3 there is at least one power of 2, 2^k, such that in base b = 2^k-1 the binomial expansion of (b+1)^floor([(n-1)*(n-2)]/k) multiplied by the remaining small power of 2 gives a palindromic expression not requiring carries in base b. James G. Merickel, Aug 05 2015 LINKS EXAMPLE a(6)=1885: the bases are 1884 (1885 is 11 in base 1884), 14 (1885 is 989 in base 14), 12 (it is 1111 in base 12), 6 (it is 12421 in base 6), and 4 (it is 131131 in base 4). CROSSREFS Cf. A007632, A007633, A029961, A029962, A029963, A029964, A029804, A029965, A029966, A029967, A029968, A029969, A029970, A029731, A097855, A099165, A087155, A171701, A171702, A171703, A171704, A171705, A171706, A171740, A171741, A171742, A253294. Sequence in context: A077201 A196467 A219506 * A260227 A260226 A101149 Adjacent sequences:  A171772 A171773 A171774 * A171776 A171777 A171778 KEYWORD nonn,base,hard,more AUTHOR James G. Merickel, Dec 18 2009 EXTENSIONS a(7) and a(8) added by James G. Merickel, Feb 04 2010 Offset changed to 1, with corresponding addition of a(1) by James G. Merickel, Jul 24 2015 Comment corrected and explained.James G. Merickel, Aug 05 2015 Definition and example rewritten by N. J. A. Sloane, Aug 05 2015 STATUS approved

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Last modified July 25 23:39 EDT 2021. Contains 346294 sequences. (Running on oeis4.)