A258188 Smallest multiple of 7 not appearing earlier that ends with n.

21, 42, 63, 14, 35, 56, 7, 28, 49, 210, 511, 112, 413, 714, 315, 616, 217, 518, 119, 420, 721, 322, 623, 224, 525, 126, 427, 728, 329, 630, 231, 532, 133, 434, 735, 336, 637, 238, 539, 140, 441, 742, 343, 644, 245, 546, 147, 448, 749, 350, 651, 252, 553, 154

Offset: 1

Author

Eric Angelini and Reinhard Zumkeller, May 23 2015

Keywords

• nonn
• base

Comments

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

The sequence is a permutation of the positive multiples of 7. - Vladimir Shevelev, May 24 2015

A258329(n) = a(n) / 7 is a permutation of the positive integers. - Reinhard Zumkeller, May 27 2015

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..9999

Crossrefs

Cf. A008589, A258083, A258217, A258329.

Programs

Haskell

import Data.List (isPrefixOf, delete)
a258188 n = a258188_list !! (n-1)
a258188_list = f 1 $ tail $ zip
   a008589_list $ map (reverse . show) a008589_list where
   f x ws = g ws where
     g ((u, vs) : uvs) = if isPrefixOf xs vs
                         then u : f (x + 1) (delete (u, vs) ws) else g uvs
     xs = reverse $ show x

Mathematica

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 *)