Context:
I'm unfamiliar with some of the symbols depicted below - specifically what appears after (Da). That right angle above the n. Also, what are the two dots above the a?
Preceding text:
Decreasing Annuities with Terms in Arithmetic Progression
The $n$-year unit decreasing annuity-immediate has $n$ payments: $n, n-1, \dots , 1$, payable at the end of each year (at times $1, 2, 3, \dots, n$). Its present value is denoted by $(Da)n⌝$.
Disclaimer:
I don't have a solid background in math. I was just reading this book and saw this formula.
Note:
The above text is from: ACTEX FM 2017 Study Manual.
This is standard actuarial notation and covered in the wikipedia page linked by "lulu" above. These symbols just abbreviate values of certain cash-flows. The first equation expresses the fact that a decreasing annuity can be written as a constant annuity minus an increasing one. Double dots have nothing to do with German umlauts, they just mean "paid in advance" in contrast to the same letter without double dots "paid in arrears" and so on ...
The right angle means that the annuity is paid for a fixed period (instead of until death). $i$ is the technical interest rate and $\nu$ the applicable discount factor.