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 A260864 Base-14 representation of a(n) is the concatenation of the base-14 representations of 1, 2, ..., n, n-1, ..., 1. 2
 0, 1, 225, 44521, 8732025, 1711559641, 335466848025, 65751518430361, 12887297839395225, 2525910379700086681, 495078434465717705625, 97035373155903680328601, 19018933138565843484771225, 3727710895159027432980276121, 10228838696316240496325238416281 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS See A260343 for the bases b such that A260851(b) = A_b(b) = b*r + (r - b)*(1 + b*r), is prime, where A_b is the base-b sequence, as here with b=14, and r = (b^b-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 = 14, we have a(n) = R(14,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) = (14+1)^2 = 14^2 + 2*14 + 1 = 121_14, concatenation of (1, 2, 1). a(15) = 123456789abcd101110dcba987654321_14 is the concatenation of (1, 2, 3, ..., 9, a, b, c, d, 10, 11, 10, d, ..., 1), where "d, 10, 11" are the base-14 representations of 13, 14, 15. PROG (PARI) a(n, b=14)=sum(i=1, #n=concat(vector(n*2-1, k, digits(min(k, n*2-k), b))), n[i]*b^(#n-i)) CROSSREFS Base-14 variant of A173426 (base 10) and A173427 (base 2). See A260853 - A260866 for variants in other bases. For primes see A261408. Sequence in context: A061051 A036428 A183822 * A265420 A171109 A239478 Adjacent sequences:  A260861 A260862 A260863 * A260865 A260866 A260867 KEYWORD nonn,base AUTHOR M. F. Hasler, Aug 01 2015 STATUS approved

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Last modified September 28 09:32 EDT 2021. Contains 347714 sequences. (Running on oeis4.)