Number of Legal Go Positions: 171 digits long

Source: Github, date indeterminate

The 361 points on a 19×19 Go board can be colored empty, black, or white. Only some of the 3^361 possible positions are legal, namely those where every group of connected stones of the same color has an empty point adjacent to it.

In the position above, black stones at E18 and N9 lack such “liberties”, making the position illegal. Due to its capture rule, the positions that can arise in a game of Go are exactly the legal positions. On Jan 20, 2016, the number of legal positions on a standard size Go board was determined to be

L19 = 208168199381979984699478633344862770286522453884530548425639456820927419612738015378525648451698519643907259916015628128546089888314427129715319317557736620397247064840935

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