The plural of calculus is calculi or calculuses. Calculare might be an autocorrection for a different language (https://en.wiktionary.org/wiki/calculare https:/

The plural of calculus is calculi or calculuses. Calculare might be an autocorrection for a different language (https://en.wiktionary.org/wiki/calculare https://en.wiktionary.org/wiki/calculare), though given that the author has a Korean name, it’s more likely just a weird typo.