When editing text on canvas, press Ctrl+U, then type the Unicode code point of the symbol you need. A preview of the symbol is shown in the statusbar. When done, press Enter. A list of Unicode codes can be found at unicode.org; for example, the integral sign character is "222b". You must have a font installed on your system that has this character; otherwise what you'll see is a placeholder rectangle. When editing text on the Text tab of the Text and Font dialog, you can use any GTK input modes that your GTK installation supports. Consult GTK documentation for details. There is also an extension called textext, that allows you to include LaTeX typeset formulas in inkscape. When saving your work in eps format, we recommend to set the option "convert text to path" in the export dialog box, to preserve the symbol.