to emulate function keys F1 - F12 on a standard US keyboard (equaling your Logitech K700 design) I suggest to do the following:
first create some file e.g. '/etc/X11/Xmodmap' with these contents:
keycode 66 = Mode_switch
keycode 10 = 1 exclam F1
keycode 11 = 2 at F2
keycode 12 = 3 numbersign F3
keycode 13 = 4 dollar F4
keycode 14 = 5 percent F5
keycode 15 = 6 asciicircum F6
keycode 16 = 7 ampersand F7
keycode 17 = 8 asterisk F8
keycode 18 = 9 parenleft F9
keycode 19 = 0 parenright F10
keycode 20 = minus underscore F11
keycode 21 = equal plus F12
second do a
xmodmap /etc/X11/Xmodmap
this will map a 'caps lock' key pressed together with some '1' - '=' key to F1 - F12.
Certainly you may define any other mode switch key than 'caps lock' for this. I personally like to use 'caps lock' for such purposes because I consider it absolutely useless if used in it's traditional way.
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and+
keys? – Lekensteyn Jul 9 '13 at 20:29