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 A260865 Base-15 representation of a(n) is the concatenation of the base-15 representations of 1, 2, ..., n, n-1, ..., 1. 2
 0, 1, 256, 58081, 13075456, 2942086081, 661970995456, 148943498386081, 33512287502995456, 7540264693665886081, 1696559556157202995456, 381725900136606353386081, 85888327530754964702995456, 19324873694420145086040886081 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS See A260343 for the bases b such that B(b) = A_b(b) = b*c + (c - b)*(1 + b*c), is prime, where A_b is the base-b sequence, as here with b=15, and c = R(b,b) =  (b^n-1)/(b-1) is the base-b repunit of length b. LINKS D. Broadhurst, Primes from concatenation: results and heuristics, NmbrThry List, August 1, 2015 FORMULA For n < b = 15, we have a(n) = A_b(n) = R(b,n)^2, where R(b,n) = (b^n-1)/(b-1) are the base-b repunits. EXAMPLE a(0) = 0 is the result of the empty sum corresponding to 0 digits. a(2) = (15+1)^2 = 15^2 + 2*15 + 1 = 121_15, concatenation of (1, 2, 1). a(16) = 123456789abcde101110edcba987654321_15 is the concatenation of (1, 2, 3, ..., 9, a, ..., e, 10, 11, 10, e, d, ..., 1), where "e, 10, 11" are the base-15 representations of 14, 15, 16. PROG (PARI) a(n, b=15)=sum(i=1, #n=concat(vector(n*2-1, k, digits(min(k, n*2-k), b))), n[i]*b^(#n-i)) CROSSREFS Base-15 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases. Sequence in context: A237068 A204302 A264096 * A205173 A205416 A229103 Adjacent sequences:  A260862 A260863 A260864 * A260866 A260867 A260868 KEYWORD nonn,base AUTHOR M. F. Hasler, Aug 01 2015 STATUS approved

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Last modified February 25 11:26 EST 2020. Contains 332233 sequences. (Running on oeis4.)