[Tex/LaTex] Slanted greater than and equal to, less than and equal to

Question

Need to produce a "Greater than" or "Equal to" symbol with a slanted equal component of the symbol, i.e. the bottom bar not parallel to the typed line, but parallel to the bottom bar of the right facing arrow. \geq begets a normal "greaterthan"/"equal to"-symbol. I've found on the internet and then tried \geqslant, but it does not work. I get an error message:

! Undefined control sequence.
l.1171 …frac{k^{n-1}_1}{k} \right) , n \geqslant
1
?

Am I missing something in the preamble?

I assume there is a similar way to achieve a Less than and equal to slant.

Answer 1

You need the amssymb package in order to use the \leqslant and \geqslant symbols.

\documentclass{article}
\usepackage{amssymb}
\begin{document}
\[ a-b\geqslant 0 \iff a \geqslant b \iff  b \leqslant a.\]
\end{document}