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Smallest multiple of 7 not appearing earlier that ends with n.
4

%I #20 Feb 07 2018 12:14:37

%S 21,42,63,14,35,56,7,28,49,210,511,112,413,714,315,616,217,518,119,

%T 420,721,322,623,224,525,126,427,728,329,630,231,532,133,434,735,336,

%U 637,238,539,140,441,742,343,644,245,546,147,448,749,350,651,252,553,154

%N Smallest multiple of 7 not appearing earlier that ends with n.

%C a(10*n) = 10*a(n).

%C The sequence is a permutation of the positive multiples of 7. - _Vladimir Shevelev_, May 24 2015

%C A258329(n) = a(n) / 7 is a permutation of the positive integers. - _Reinhard Zumkeller_, May 27 2015

%H Reinhard Zumkeller, <a href="/A258188/b258188.txt">Table of n, a(n) for n = 1..9999</a>

%t a[n_] := a[n] = For[k = 7, True, k = k + 7, If[Divisible[k - n, 10^IntegerLength[n]] && FreeQ[Array[a, n-1], k], Return[k]]]; Array[a, 54] (* _Jean-François Alcover_, Feb 07 2018 *)

%o (Haskell)

%o import Data.List (isPrefixOf, delete)

%o a258188 n = a258188_list !! (n-1)

%o a258188_list = f 1 $ tail $ zip

%o a008589_list $ map (reverse . show) a008589_list where

%o f x ws = g ws where

%o g ((u, vs) : uvs) = if isPrefixOf xs vs

%o then u : f (x + 1) (delete (u, vs) ws) else g uvs

%o xs = reverse $ show x

%Y Cf. A008589, A258083, A258217, A258329.

%K nonn,base

%O 1,1

%A _Eric Angelini_ and _Reinhard Zumkeller_, May 23 2015