Hence the original premise, that 13 is odd, is correct. Complexity Rates of Growth A most important factor in determining what computers can and cannot do is concerned with the complexity of a program. The concept of complexity is not to do with how hard an algorithm is to follow or how simple an algorithm looks on paper. The concept of complexity deals with the number of instructions that a computer has to carry out in order to solve a problem. Most algorithms use some kind of iteration or recursion to solve a problem.
Complexity is concerned with the number of iterations, the depth of the recursion and so on.
A more complex algorithm will use a deeper level of recursion and hence take a longer time to process. The important thing to make note of is that there is a limit to how complex an algorithm can become and still remain useful. Generally, programs with constant and linear levels of complexity are very fast. Polynomial complexity is generally acceptable for most problems. Exponential complexity is almost always too slow for it to be of any use, and factorial or higher complexity is completely useless except for the most simple problems.