beyond a doubt vs beyond doubt

beyond a doubt is not an adjective while beyond doubt is an adjective.

Word	Noun	Adjective	Verb	Adverb
beyond a doubt	No	No	No	Yes
beyond doubt	No	Yes	No	Yes

As adverbs, beyond a doubt and beyond doubt are synonyms defined as:

beyond a doubt and beyond doubt: in a manner or to a degree that could not be doubted

Other synonyms of beyond a doubt include beyond a shadow of a doubt, indubitably.

beyond a doubt (adjective)	beyond doubt (adjective)
	too obvious to be doubted

beyond a doubt (adverb)	beyond doubt (adverb)
in a manner or to a degree that could not be doubted	in a manner or to a degree that could not be doubted