User blog comment:MilkyWay90/Help with understanding Veblen array notation/@comment-30754445-20180811202716/@comment-35392788-20180818055811

It should be sup(X_n) where X_1 = 0 and X_(n+1) = phi(X_n,0)

phi(1,0,0) = phi(phi(phi(...),0),0)