# What does this matrix representation mean?

What exactly is d1 here? Is it a particular row? Or is it the column? Why is a singular vector from the matrix multiplying out with each element inside the r vector?

• Please take the time to type the crucial parts of your question in as text instead of pasting an image, which is neither searchable nor accessible to people using screen readers. You can find a MathJax tutorial for formatting your mathematical expressions here.
– amd
Oct 18, 2017 at 18:35
• @amd thank you. I have been looking for this tutorial for a bit.
– SDG
Oct 18, 2017 at 20:11

• $d_{1j}$ is the $(1,j)$-th entry of the matrix $D$.
• You would not lose consistency by interpreting $d_1$ as the first row of $D$, if you so wish. That I know of, calling $d_{ij}$ the $j$-th element of the $i$-th column would be rather uncanny.
• The author just wrote the formula of the first entry of the column vector $D\cdot r$.