sample formula

I just do not understand the $C$ with the small $D$ means. I do not care what the formula is for, but why are the small letters next to the big letters and not just their own variable??

  • 3
    $\begingroup$ That C has a subscript of d. Subscripts carry different meanings depending on context. $\endgroup$ Mar 5, 2019 at 0:07
  • 2
    $\begingroup$ Sometimes, people need constants, they are very happy to use something like $C$, $C_1$, $C_2$, $C_3$, or $C'$, $C''$ or the same using the $K$... But sometimes, all these constants are not enough, or we have some "objects" $D,E,F$, and there should be some appropriate rôle of some constants for them, then people use $C_D$, $C_E$, $C_F$ to make the parallel more transparent. (Or less, well it depends, some authors use too much magic like this...) At any rate, the "sample formula" linked above is not part of mathematics, which never leaves the beauty, and never creates monsters like that... $\endgroup$
    – dan_fulea
    Mar 5, 2019 at 0:11

1 Answer 1


The small letters are called indexes, but what they mean is highly dependenent on the context.

For example:

Say you want to have $C$ denote the cost of something. Now say you would like to have two different costs involved in the same equation, the cost of the daily rental and the cost of food for a day. In this case you may write $C_R$ for rental cost and $C_F$ for food cost to be able to differentiate between the two $C$s.

You could also try to say that $C,D$ depends on the same thing say $t$. Then you could index them with $t$ stressing that they are a sort of function depending on the variable $t$.

As I said it is highly dependent on the context so you should give some more information about what you seek.


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