OFFSET
1,3
COMMENTS
Given an integer -1 < k < 10, n is a Belgian-k number if an infinite sequence in ascending order can be constructed starting at k and including n, and the first differences of that sequence give the base 10 digits of n repeatedly and no others.
Mauro Fiorentini (see Angelini link) explains that all base 10 Harshad numbers (A005349) are also Belgian-0 numbers. - Alonso del Arte, Feb 13 2014
Every integer in this sequence is also a Belgian-k number, where k is the sum of digits of the integer. - Davide Rotondo, Jun 12 2024
LINKS
Vincenzo Librandi and Reinhard Zumkeller, Table of n, a(n) for n = 1..10000, first 1000 terms from Vincenzo Librandi
Eric Angelini, Belgian numbers.
Eric Angelini, Belgian Numbers [Cached copy with permission]
EXAMPLE
13 is a Belgian-0 number because of the sequence
0, 1, 4, 5, 8, 9, 12, 13, 16, 17, 20, ...
the first differences of which are
1, 3, 1, 3, 1, 3, 1, 3, 1, 3, ...
176 is a Belgian-0 number because, starting from 0 (the seed), one can build a sequence containing 176 in this way:
0.1.8.14.15.22.28.29.36.42.43.50.....155.162.168.169.176.... (sequence)
.1.7.6..1..7..6..1..7..6..1..7..........7...6...1...7.. (first differences)
14 is not a Belgian number because, although we can construct a sequence with the required starting point and the required first differences (namely 0, 1, 5, 6, 10, 11, 15, ...), that sequence does not contain 14.
MATHEMATICA
belgianQ[n_, k_] := If[n < k, False, Block[{id = Join[{0}, IntegerDigits@ n]}, MemberQ[ Accumulate@ id, Mod[n - k, Plus @@ id]] ]]; Select[ Range@ 120, belgianQ[#, 0] &] (* Robert G. Wilson v, May 06 2011 *)
PROG
(Haskell)
a106039 n = a106039_list !! (n-1)
a106039_list = filter belge0 [0..] where
belge0 n = n == (head $ dropWhile (< n) $
scanl (+) 0 $ cycle ((map (read . return) . show) n))
-- Reinhard Zumkeller, May 07 2015
CROSSREFS
KEYWORD
base,easy,nonn
AUTHOR
Eric Angelini, Jun 07 2005
EXTENSIONS
Offset changed by Reinhard Zumkeller, May 08 2015
STATUS
approved