I am currently working through this document describing the calculation of an accurate Geopotential model.
In equation 6.15, there is a double summation. The first summation specifies summation for all values of f (frequencies of waves in this case), and the second indicates that, for every frequency, we should perform a summation of the following expression for + and -. The expression includes multiple sets of +- signs, with one of them being actually a -+ sign.
My question is, does this summation for + and - mean to perform a summation of 2 terms, the first one obtained by taking the first sign in each +- sign (i.e., for the case of the -+ sign, it would be a -), and the second one obtained by taking the second sign in each +- sign (i.e., a + for the -+ sign)?
Editing to leave the specific formula (I am including a simplified to show only the notation I'm having trouble with):
$$\sum_{+}^- (\mathcal{C}^\pm \mp i\mathcal{S}^\pm)e^{\pm i\theta}$$