OFFSET
1,4
COMMENTS
LINKS
R. Zumkeller, Table of n, a(n) for n = 1..10000
R. Zumkeller, Example: n=42
EXAMPLE
Proper divisors for n=21 are: 1, 3, and 7:
a(39) = #{[1,3,7], [1,7,3], [3,1,7], [3,7,1], [7,1,3], [7,3,1]} = 6;
proper divisors for n=12 are: 1, 2, 3, 4, and 6:
a(12) = #{[2,3,4,1,6], [4,3,2,1,6], [6,1,2,3,4], [6,1,4,3,2]} = 4;
proper divisors for n=42: 1, 2, 3, 6, 7, 14, and 21:
a(42) = #{[2,21,1,6,7,3,14], [2,21,1,14,3,7,6], [3,14,1,6,7,2,21], [3,14,1,21,2,7,6], [6,1,14,3,7,2,21], [6,1,21,2,7,3,14], ...} = 36, see the appended file for the list of all permutations.
CROSSREFS
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, May 24 2010
STATUS
approved