I came up with this idea about a year ago, I think, but a conversation just now jogged my memory and so I thought I'd check.

Shusaku played against Shuho, Shuho played against Kita Fumiko, and Sugiuchi Kazuko studied under Kita, so (if they played together) she should have the number three.

But do we have evidence that Sugiuchi indeed played against Kita? And does any other surviving person have a claim to a three-step number?

All the other candidates listed on SL's Shusaku number article as having S3 seem to have passed away, some many years ago: all of Kita's other students (perhaps - not all their articles have a date of death, or indeed retirement); Iwamoto, Go Seigen, Kitani, Segoe, Miyashita Shuyo, Felix Dueball, Kikuchi Yasuro (recently, ofc) and Hashimoto Utaro.

Also, would it be trivial to assign every professional a maximum Shusaku number by designing a script to search the game trees in a database?

Where is the evidence that Kita played Shuho?

This is where ez4u would come in handy! Although I imagine the 日本棋院 people would be perplexed by someone randomly asking them whether 杉内先生 played Kita sensei 。。。

@JohnFairbairn "Where is the evidence that Kita played Shuho?"

This claim was made in 2009 by American Go book translator Bob McGuigan (with whom I expect you're familiar) on his SL homepage, who claimed a Shusaku path Shusaku --> Shuho --> Kita --> Shiratori Sumiko --> himself. I assumed that it was based on kifu or other evidence that I didn't have access to, eg. from GoGoD.

It is possible but unlikely in view of the dates.
This missing link can be repaired using Iwasaki Kenzo, who played both Shusaku and Kita.

Since you ask, and it is April 1st, I did something. See shusaku-number.html. Corrections and improvements are welcome.

Hey, you have interesting typo in top (ie 2nd line) of this page https://homepages.cwi.nl/~aeb/go/games/ ... index.html

Thanks! Fixed.

I have a Shusaku number of 6! (or less)

It should be possible to compute an upper bound of the Shusaku number using the European Go Database, and knowing that some European players on https://homepages.cwi.nl/~aeb/go/games/ ... umber.html have Shusaku number 4 or 5.

Based on real-life tournament games, my Shusaku number is probably 7. If we add friendly games it's 6 or less.