Neue Rechtschreibung changed numerieren to nummerieren. This is not optional, so numerieren is incorrect now. How come numerisch did not change?
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This is said to be justified by the Stammprinzip: words sharing the same root shouldn't diverge in matters of orthography. Strengthening the Stammprinzip was said to be one of the "pro Rechtschreibreform" arguments. In addition, the "u" as a short stressed vowel, would require the following consonant to be duplicated.
So, since "nummerieren" is said to be derived from "Nummer", it should be written in the same way, with double "m".
There's a lot of criticisms about that point: since "nummerien" as well as "Nummer" and "Numerik" shares the same latin root "numerus" (verb: "numerare"), the Stammprinzip would urge "numerieren" if applied inconsistently. Nevertheless, "nummerieren" is in fact the sole correct form of spelling.
EDIT: However, this doesn't justify why only "nummerieren" was assimilated to "Nummer" but not "numerisch". According to canoo.net, "numerisch" is derived from "Nummer", whereas "Numerik" is derived from "numerisch". So "Numerik" can't act as root to apply the "Stammprinzip" on "numerisch".
According to the "amtliche Regelung der deutschen Rechtschreibung"
it is clear that in the case of "nummerieren" there is an exception from the rule that there is a doubling of then consonant when a short vowel follows (as is the case in "Nummer" but not in the case of "nummerieren"). This exception holds true when a resulting word stems from a word where consonant doubling occurs even when the pronunciation of the resulting word is different.
"Numerisch" on the other hand is a direct loanword derived from Latin numericus where a doubling does not occur. As this is unclear we will also find "nummerisch" in written German (but this is not done according to the "amtliche" rules and should therefore be discouraged)
Everything said above is correct - but one thing is missing: