Chihiro numbers^{[1]} 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.

The original, now deleted,^{[2]}^{[3]} Wikipedia article was posted for a week and was even linked from the front page. Despite this, the page was outed as a hoax for its name not being properly cited. However, due to the publicity of reaching Wikipedia's front page, the name still is popular when describing the sequence as a whole.

This sequence is equal to the sequence of booga-, and closely related to the Ackermann function.

The first values are 2, 4 and 27. The next value, \(4 \uparrow\uparrow 4\), is too large to write out (it is about equal to \(10^{8.07230472603 \times 10^{153}}\)). The original article, as well as the description from the front page, described them as "one of the fastest-growing sequences known to mathematicians", which is only true when one compares the Chihiro numbers to non-googological functions.

The article described the sequence as first being named by the fictitious mathematician Isoroku Kagachi after his son, who was also a mathematician. (Note that Chihiro is usually a female name in Japanese, but can also be used for males.)