A test for Belgian-ity
Eugene McDonnell
eemcd at mac.com
Wed Jun 15 21:35:48 CEST 2005
More or less empirically, I have found that I can make an easy test to
see if a given positive integer is Belgian; I'd appreciate it if
someone more versed in mathematics could prove or disprove the
following, before I pollute the Encyclopedia with an incorrect comment
for A106039:
To test the Belgian-ity of a positive integer n, find s, the cumulative
sum of its digits, and m the sum of its digits; then n is Belgian if
there is a 0 in any of the differences of n minus the items of s, mod
m. For example, if n is 176 then s is 1 8 14, and m is 14; 176 - 1 8 14
is 175 168 162, and the residues mod(14) are 7 0 8, so 176 is Belgian.
Contrariwise, if n is 177, s is 1 8 15, and m is 15; 177 - 1 8 15 is
176 169 162, and the residues, mod(15) are 11 4 12, so 177 is not
Belgian.
A more exotic example:
Let n be 1234567898765; then s is 1 3 6 10 15 21 28 36 45 53 60 66 71,
and m is 71.
(1234567898765 - 1 3 6 10 15 21 28 36 45 53 60 66 71) mod(71) is:
20 18 15 11 6 0 64 56 47 39 32 26 21
which contains a 0, so n is Belgian.
Eugene McDonnell
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