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 A260853 Base-3 representation of a(n) is the concatenation of the base-3 representations of 1, 2, ..., n, n-1, ..., 1. 20
 0, 1, 16, 439, 35350, 2864599, 232046890, 18795930559, 1522471570630, 369960528437035, 269701223137448146, 196612191672080116867, 143330287729139571972130, 104487779754548024866115515, 76171591441065652665051372946, 55529090160536864641400481743827 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS The base 3 is listed in A260343, which means that a(3) = A260851(3) = 439 = 121021_3 is prime and therefore in A260852. See these sequences for more information. LINKS D. Broadhurst, Primes from concatenation: results and heuristics, NmbrThry List, August 1, 2015 EXAMPLE a(0) = 0 is the result of the empty sum corresponding to 0 digits. a(2) = 16 = 121_3 is the concatenation of (1, 2, 1). a(3) = 439 = 121021_3 is the concatenation of (1, 2, 10, 2, 1), where the middle "10" is the base-3 representation of 3. MATHEMATICA Join[{0}, Table[FromDigits[Join[Flatten[IntegerDigits[Range[n], 3]], Flatten[ Reverse[ Most[ IntegerDigits[Range[n], 3]]]]], 3], {n, 20}]] (* Harvey P. Dale, Mar 11 2019 *) PROG (PARI) a(n, b=3)=sum(i=1, #n=concat(vector(n*2-1, k, digits(min(k, n*2-k), b))), n[i]*b^(#n-i)) CROSSREFS Base-3 variant of A173426 (base 10) and A173427 (base 2). See A260854 - A260866 for variants in other bases b = 4, ..., 16. Sequence in context: A223686 A000489 A075852 * A068792 A229583 A241077 Adjacent sequences:  A260850 A260851 A260852 * A260854 A260855 A260856 KEYWORD nonn,base AUTHOR M. F. Hasler, Aug 01 2015 STATUS approved

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Last modified December 8 18:37 EST 2019. Contains 329865 sequences. (Running on oeis4.)