dogged, dour, persistent, pertinacious, tenacious, unyielding -- (stubbornly unyielding; "dogged persistence"; "dour determination"; "the most vocal and pertinacious of all the critics"; "a mind not gifted to discover truth but tenacious to hold it"- T.S.Eliot; "men tenacious of opinion")
unyielding -- (resistant to physical force or pressure; "an unyielding head support")
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