Now we present the main result, that is (that is to say), the achievable region...
Could you please tell me if it's obigatory to put a comma after "that is" ( and "that is to say") or we can skip it?
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The comma is used for setting off nonessential explanations. If an explanation or definition occurs as an appositive it should be set off with commas:
When the nonessential explanation is not at the end of the sentence, like in your example, it should be set off with a pair of commas.
If the nonessential explanation is placed at the end of the sentence then only one comma is needed.