A formal written expression used to embed a yes/no question inside a sentence. Often follows a plain-form clause. In spoken Japanese, ～かどうか is more common.
We still don't know whether or not that information is correct.
Please let us know whether or not you will attend as soon as possible.
Standard kanji-kana expression; the kanji 否 is read いな here.
Combination of the question particle か, the classical negative 否 (いな, 'no'), and another か. The pattern is a fixed formal expression.