Ackermann’s Function Index   <<   >>
  

ack←{
  0=⍺: 1+⍵
  0=⍵: (⍺-1) ∇ 1
  (⍺-1) ∇ ⍺ ∇ ⍵-1
}

   2 ack 3
9
   3 ack 2
29