Is writing "Salam" in a greeting correct, and if not, is it obligatory to respond to the one who says it?

The Sunnah regarding greeting is for the Muslim to say: "Assalamu Alaikum" (Peace be upon you). It is also permissible to say: "Salam Alaikum" (Peace upon you) in an indefinite form. As for limiting oneself to the word "Salam" (peace) for greeting and supplication, there is no objection to it, as the angels did with Ibrahim (peace be upon him). And whoever is greeted with this formula, it is obligatory to return the greeting with a similar or better one, because returning the greeting is obligatory.