Author: Stan Eisenstat
Subject: Re: [Cs223] Lower triangular matrix
Date: Tuesday, 03 Mar 2020, 20:17:14

    > Message Posted By: Unknown
    >
    > Could you explain the meaning of the "(n-j+2)" term in the "n + (n-1) +
    > ... + (n-j+2)" line of the lower triangular matrix problem solution?

n-j+2 is the number of nonzeroes in column j-1.
=====

    > Also, could you explain how you go from this
    > n + (n-1) + ... + (n-j+2)
    > to this
    > (2n-j+2)/2 * (j-1)
    > to this
    > (first term + last term)/2 * #term?

The last line is a general rule for added an arithmetic
progression such as n + (n-1) + ...  + (n-j+2).  The
second line applies this rule to the first line.

--Stan-