Digital roots were known to Pythagoras. According to https://medium.com/@beeezy/pythagoras-hacked-your-password-802a027796ed https://medium.com/@beeezy/pythagor

Digital roots were known to Pythagoras. According to https://medium.com/@beeezy/pythagoras-hacked-your-password-802a027796ed https://medium.com/@beeezy/pythagoras-hacked-your-password-8... they're also mentioned in "Indian Vedic math". Seems like prior art for a numeric hash.