OFFSET
0,7
COMMENTS
a(n) is the number of 3-element subsets (i<j<k) of (0,...,n-1) such that both i,j,k and a(i),a(j),a(k) are arithmetic progressions (including the case a(i)=a(j)=a(k)). That is, k-j=j-i>0 and a(k)-a(j)=a(j)-a(i).
The sequence appears to grow faster than n but slower than n^(1+c) for any positive c.
EXAMPLE
For n=7, the triples (0,1,2),(0,3,6),(2,4,6),(3,4,5) satisfy the stated conditions, so a(7)=4
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alex Abercrombie, Jan 06 2011
STATUS
approved