OFFSET
1,2
COMMENTS
If pairs are restricted to b<c, we get the variant 0, 0, 0, 1, 1, 1, 1, 3, 4, 4, 4, 5, 5, 5, 5, 8, 8, 10, 10, 11, 11, 11, 11, 14, 15,...
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
P. Erdos and T. Motzkin, Density of pairs with same prime factors, Am. Math. Month. vol 97 no 10 (1990) p 937, problem 5735.
EXAMPLE
a(16)=24 counts the 16 pairs (b,b) with 1<=b<=16 plus the 8 pairs (2,4), (2,8), (2,16), (4,8), (4,16), (8,16), (3,9), (6,12).
PROG
(PARI) samepf(m, n)=my(g=gcd(m, n), t=g); m/=g; while((t=gcd(t, m))>1, m/=t); if(m!=1, return(0)); t=g; while((t=gcd(t, n))>1, n/=t); n==1
a(n)=sum(b=1, n, sum(c=b, n, samepf(b, c))) \\ Charles R Greathouse IV, Jan 09 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
R. J. Mathar, Jul 11 2008
STATUS
approved