The triangular number $T_n$ is the total number of objects arranged in a triangle of $n$ rows, i.e. $T_n:=\dfrac{n(n+1)}{2}$. Then, one might ask for the triangular root of a number $x$, as the positive number $n$ (not necessarily an integer) such that $T_n=x$. Clearly, $n=\dfrac{\sqrt{8x+1}-1}{2}$, but:
Question(s): Is there any reasonably standard notation for this root? If needed, would perhaps $\sqrt[^\Delta]{x}$ be appropriate?
It seems like a very simple question, but I couldn't seem to find any. Wikipedia suggests none as far as I can see, and following Wikipedia's reference to Euler's Elements of Algebra I could not seem to find where they where even defined, but I may have missed it. Also, I am very unfamiliar with number theory, and most searches seem to result in material related to cube or even higher $n$:th roots.
The OEIS uses $\operatorname{trinv}(x)$ for $\left\lfloor\frac{\sqrt{8x+1}+1}2\right\rfloor$, which differs from your function by $1$; it appears 43 times as of this post's writing.