# “undefined control sequence” error with curly brackets

I'm writing a math equation inside which I have to express curly brackets. What I intend to write should look like {h_n}^infinity_0

My code is:

\par
The problem asks for estimates of the sizes of the coefficients $\{h_{n}\}\limit_{0}^{\infty}$.


While the preview shows this as exactly what I want, when I try compiling it I get an error message that calls it an "undefined control sequence".

I don't understand why: could anyone help explain this?

• You are probably looking for \limits, not \limit. – Werner Feb 28 '15 at 19:22

I wouldn't use \limits in this case since it'll end up placing the lower and upper limits of the sequence below and above the right-hand curly brace -- probably not what you intended.
If you write \{h_{n}\}_{0}^{\infty}$ (see the middle example below, labelled "not awful"), the upper and lower limits may look like they're too close to each other, especially in text-style math mode. One way to fix this is to encase the closing curly brace, \}, in a pair of braces to change its status from "math-close" to "math-ordinary"; doing so will loosen the spacing quite a bit. It may be best, though, to define a custom macro called, say, \seqwlimits to standardize the task at hand and to assure adequate vertical separation of the upper and lower limits. \documentclass{article} \newcommand\seqwlimits[3]{\{#1\}_{\mathstrut#2}^{\mathstrut#3}} \begin{document} \begin{tabular}{ll} huh?? &${\{h_{n}\}}\limits_{0}^{\infty}$\\[2ex] awful &$\{h_{n}\}\limits_{0}^{\infty}$\\[2ex] not awful&$\{h_{n}\}_{0}^{\infty}$\\[2ex] better &$\{h_{n}{\}}_{0}^{\infty}$\\[2ex] % note the extra { and } best &$\seqwlimits{h_n}{0}{\infty}\$\\

• I agree that your best solution is better than your better solution, which is better than the bad one. So, why isn't \mathstrut used by default? Is it better only with 2 scripts? – Sigur Feb 28 '15 at 19:46
• @sigur - I've modified my answer to provide two more possibilities. By default, \} (the closing curly brace) is assigned state "math-close" by TeX, and any subsequent sub- and superscripts are set in very tight mode by design. One way to overcome the cramped look is to change the state of the closing curly brace to "math-ordinary", e.g., by encasing it in curly braces (see the case labelled "better" in the example). To get even better spacing, IMNSHO, I suggest using \mathstrut directives, which insert invisible objects with height and depth of parentheses. – Mico Feb 28 '15 at 20:10