For n=2, the largest number not of the form k_1+k_2 with k_1>=1, k_2>=1 is 1! So a(2) = 1. For n=3 the largest number not of the form ab+bc+ca (a,b,c >= 1) is believed to be 462 (see A025052).
CROSSREFS
See A025052 for numbers of form ab+bc+ca (which is the case n=3), A027563 for n=4, A027564 for n=5. See also A027566.
