OFFSET
1,7
LINKS
EXAMPLE
a(12)=9: Such as 12 trees in 9 rows: example for 12 trees with 9 rows: ABCD,AEFG,AHIJ,BEHK,BFIL,CEJL,CGIK,DFJK,DGHL. Let t^2+t+1=0, coordinate in projective plane: A[0:1:0], B[1:0:0], C[1:1+t:0], D[1:t:0], E(0,0), F(0,1), G(0,-t), H(1,0), I(1,1), J(1,t+1), K(1+t,0), L(-t,1).
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Zhao Hui Du, Feb 07 2010
EXTENSIONS
a(12) corrected by Zhao Hui Du, Oct 30 2019
STATUS
approved