User blog comment:Vel!/Ordinals and arrow notation/@comment-3427444-20131212014842/@comment-3427444-20131212064217

ε(n,m) is Sbiis Saibian's extension of the epsilon function. Here are some values:

...
 * ε(a,0) = phi(1,a)
 * ε(a,1) = phi(2,a)
 * ε(a,2) = phi(3,a)
 * ε(a,w) = phi(w,a)