See:
https://scicomp.stackexchange.com/questions/23202/what-are-differences-between-a-priori-and-posteriori-error-estimate-in-numer
https://qastack.cn/scicomp/23202/what-are-differences-between-a-priori-and-posteriori-error-estimate-in-numer
See:
https://scicomp.stackexchange.com/questions/23202/what-are-differences-between-a-priori-and-posteriori-error-estimate-in-numer
https://qastack.cn/scicomp/23202/what-are-differences-between-a-priori-and-posteriori-error-estimate-in-numer