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 A253253 a(n) = smallest divisor of the concatenation of n and n+1 that did not occur earlier. 2
 1, 23, 2, 3, 4, 67, 6, 89, 5, 337, 8, 1213, 9, 283, 379, 7, 859, 17, 10, 43, 1061, 13, 14, 25, 421, 37, 11, 41, 293, 433, 12, 53, 1667, 15, 16, 3637, 21, 349, 20, 449, 19, 4243, 24, 35, 2273, 1549, 1187, 373, 18, 5051, 28, 51, 2677, 1091, 463, 5657, 2879, 27 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Is this a permutation of the integers > 0 ? Comment from N. J. A. Sloane, May 19 2017 (Start): It should not be difficult to prove that every positive integer appears. If not, let m be the smallest missing number. There is an n_0 such that for all n >= n_0, a(n) > m. The theorem will follow if we can find an N > n_0 such that m divides the concatenation of N and N+1. Let N have k digits and suppose that 10^(k-1) <= N <= 10^k - 2. The concatenation of N and N+1 is N*(10^k+1)+1, so we want to find numbers k and N such that N*(10^k+1) == -1 mod m. Case (i). If gcd(m,10)=1, then by Euler's theorem, 10^phi(m) == 1 mod m, so we can take k to be a sufficiently large multiple of phi(m), and then take N to be a number of the form r*m-1 in the range 10^(k-1) <= N <= 10^k - 2. Case (ii). If m = 2^r or 5^r, then for large k, 10^k+1 == 1 mod m, and we take N to be of the form m*s-1 in the range 10^(k-1) <= N <= 10^k - 2. The other cases are left to the reader. (End) LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 Éric Angelini, Divisors of the concatenation of n and n+1, SeqFan list, Jun 03 2015. PROG (Haskell) import Data.List (insert); import Data.List.Ordered (minus) a253253 n = a253253_list !! (n-1) a253253_list = f a001704_list [] where    f (x:xs) ds = y : f xs (insert y ds) where                  y = head (a027750_row' x `minus` ds) CROSSREFS Cf. A027750, A001704, A256285. Sequence in context: A040521 A040524 A306374 * A225003 A040525 A040526 Adjacent sequences:  A253250 A253251 A253252 * A253254 A253255 A253256 KEYWORD nonn,base,look AUTHOR Eric Angelini and Reinhard Zumkeller, Jun 05 2015 STATUS approved

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Last modified December 12 20:12 EST 2019. Contains 329961 sequences. (Running on oeis4.)