Chihiro numbers are numbers of the form n[n]n or equivalently n[n+1]2, where [n] represents the n-th hyper-operator. They are denoted as C(n); alternately, to minimize conflict with all the other C functions in googology, they can be denoted チ(n) with a Japanese "chi" kana, as Isoroku and Chihiro Kagachi were purported to do in the original article. This sequence is equal to the sequence of booga-, and closely related to the Ackermann function .