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authoritative 
1 accurate, authentic, definitive, dependable, factual, faithful, learned, reliable, scholarly, sound, true, trustworthy, truthful, valid, veritable 
2 assertive, autocratic, commanding, confident, decisive, dictatorial, dogmatic, dominating, imperative, imperious, imposing, lordly, masterly, peremptory, self-assured 
3 approved, authorized, commanding, legitimate, official, sanctioned, sovereign 
1 deceptive, undependable, unreliable 
2 humble, subservient, timid, weak 
3 unauthorized, unofficial, unsanctioned