OFFSET
1,2
COMMENTS
This sequence was used by LuoYuping when he set a problem for NOI 2015 Day1 Problem3.
a(n) is the number of ways to find X and Y where set X and Y are subsets of {2,...,n}, and for all a in X and all b in Y, gcd(a,b) = 1. Also note that X or Y can be empty.
REFERENCES
National Olympiad in Informatics 2015, China, Day 1 Problem 3.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..100 (first 80 terms from Giovanni Resta)
Sirius Caffrey, C++ program for A260185
Sirius Caffrey, Python program for A260185
FORMULA
a(p) = 3*a(p-1) for p prime. - Alois P. Heinz, Jul 19 2015
EXAMPLE
For n=1 the 1 pair of sets is [{},{}].
For n=2 the 3 pairs of sets are [{},{}], [{2},{}], and [{},{2}].
For n=3 the 9 pairs of sets are [{},{}], [{2},{}], [{},{2}], [{3},{}], [{},{3}], [{2,3},{}], [{},{2,3}], [{2},{3}], and [{3},{2}].
PROG
(C++) // see link above
(Python) # see link above
CROSSREFS
KEYWORD
nonn
AUTHOR
Sirius Caffrey, Jul 17 2015
EXTENSIONS
a(31)-a(32) from Giovanni Resta, Jul 18 2015
STATUS
approved