How to Program log z

While Fortran was the first high-level programming language used for scientific computing, Algol 60 was the vehicle for publishing mathematical software in the early 1960s. Algol 60 had real arithmetic, but complex arithmetic had to be programmed by working with real and imaginary parts. Functions of a complex variable were not built-in, and had to be written by the programmer.


I’ve written a number of papers on algorithms to compute the (principal) logarithm of a matrix. The problem of computing the logarithm of a complex scalar—given a library routine that handles real arguments—might appear trivial, by comparison. That it is not can be seen by looking at early attempts to provide such a function in Algol 60.

The paper

J. R. Herndon (1961). Algorithm 48: Logarithm of a complex number. Comm. ACM, 4(4), 179.

presents an Algol 60 code for computing \log z, for a complex number z. It uses the arctan function to obtain the argument of a complex number.


The paper

A. P. Relph (1962). Certification of Algorithm 48: Logarithm of a complex number. Comm. ACM, 5(6), 347.

notes three problems with Herndon’s code: it fails for z with zero real part, the imaginary part of the logarithm is on the wrong range (it should be (-\pi,\pi] for the principal value), and the code uses log (log to the base 10) instead of ln (log to the base e). The latter error suggests to me that the code had never actually been run, as for almost any argument it would produce an incorrect value. This is perhaps not surprising since Algol 60 compilers must have only just started to become available in 1961.

The paper

M. L. Johnson and W. Sangren, W. (1962). Remark on Algorithm 48: Logarithm of a complex number. Comm. CACM, 5(7), 391.

contains more discussion about avoiding division by zero and getting signs correct. In

D. S. Collens (1964). Remark on remarks on Algorithm 48: Logarithm of a complex number. Comm. ACM, 7(8), 485.

Collens notes that Johnson and Sangren’s code wrongly gives \log 0 = 0 and has a missing minus sign in one statement. Finally, Collens gives in

D. S. Collens (1964). Algorithm 243: Logarithm of a complex number: Rewrite of Algorithm 48. Comm. CACM, 7(11), 660.

a rewritten algorithm that fixes all the earlier errors.

So it took five papers over a three year period to produce a correct Algol 60 code for the complex logarithm! Had those authors had the benefit of today’s interactive computing environments that period could no doubt have been shortened, but working with multivalued complex functions is necessarily a tricky business, as I have explained in earlier posts here and here.

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