L&S lists both these verbs, viz. indĭcare,avisse,atum, & indīco, indīxisse,indictum.
Does anyone know how (if?) they are related? What is it that can explain the difference in vowel quantity? Are there other examples of this variation?
Thank you.
dĭcare/dīcere. They do have common origin, as their Greek cognate δεικ- “show” is enough to (forgive me) show, but their semantic and morphological separation is complete. Beyond that I can’t say, but stem vowels frequently show non-random quantitative variation, so I expect the bifurcation reflects a differential generalization. But I’m only guessing.
There is also the combination praedico, praedicare / praedīco, praedicere, which used to bother me when I started reading ecclesiastical Latin, since they are similar in meaning: praedicare “to proclaim, preach” praedicere “to say beforehand, prophesy”.
But this pair too of course retains the basic show/tell distinction, as all the compounds will.
dicăre never even crossed my mind. With hindsight it seems so obvious. Thanks MWH!
I hadn’t thought about that, but you’re right: pradicare implies speech but the meaning is more like “show forth, make known”, whereas praedicere is focused on the act of saying something.
Miguel, dicăre never even crossed my mind either—neither the one thing nor the other, as Winston Churchill remarked of a Mr Bossom’s name. A typo, I presume, but of which? 
Michael
Pff 
dĭcāre*