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 A309809 a(n) is the concatenation of n and 2n+1. 3
 13, 25, 37, 49, 511, 613, 715, 817, 919, 1021, 1123, 1225, 1327, 1429, 1531, 1633, 1735, 1837, 1939, 2041, 2143, 2245, 2347, 2449, 2551, 2653, 2755, 2857, 2959, 3061, 3163, 3265, 3367, 3469, 3571, 3673, 3775, 3877, 3979, 4081, 4183, 4285, 4387, 4489, 4591, 4693, 4795, 4897 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 FORMULA a(n) = n*10^(ceiling(log_10(2*n+1))) + 2*n + 1. EXAMPLE a(3)=37 is the concatenation of 3 and 7. MAPLE seq(n*10^(ceil(log[10](2*n+1)))+2*n+1, n=1..100); CROSSREFS Cf. A001704, A309808. Sequence in context: A161603 A075033 A017533 * A264732 A298483 A195558 Adjacent sequences:  A309805 A309806 A309808 * A309810 A309811 A309812 KEYWORD nonn,base AUTHOR Robert Israel, Aug 17 2019 STATUS approved

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Last modified December 15 17:03 EST 2019. Contains 330000 sequences. (Running on oeis4.)