It really depends on the program. Some problems derive most of their complexity from trying to correctly model complex real world rules, like calculating pensio

It really depends on the program. Some problems derive most of their complexity from trying to correctly model complex real world rules, like calculating pension payments for employees that were hired at different times, for different positions, under different union agreements, having worked varying numbers of years. Some problems have short, clear definitions yet remain challenging. An algorithm for solving them may have been discovered decades (or even centuries) ago. But trying to produce more efficient solutions can be a life's work and more. One of the most obvious examples is integer factorization, which had a basic algorithm published 800 years ago yet is still a research problem today: https://en.wikipedia.org/wiki/Trial_division https://en.wikipedia.org/wiki/Trial_division https://en.wikipedia.org/wiki/Integer_factorization#Factoring_algorithms https://en.wikipedia.org/wiki/Integer_factorization#Factorin...