There were four prominent cryptographers at the time, and Satoshi sent emails to three of them. Nick Szabo was the only one who didn't receive one.

Adam Back invented Hashcash in 1992.

Wei Dai came up with B-Money based on Hashcash a year later.

Hal Finney implemented RPoW in 2003.

Then, Nick Szabo conceived Bit Gold.

nostr:note1j356ds3p7sr07l5q5ag4ngpc0p8rvjnx53nazkk6hyyh5lsue62qmra5y5