Pertinacious
Pertinacious , adjective
[Latin pertinax, -acis; per + tenax tenacious. See Per-, and Tenacious.]
1.
Holding or adhering to any opinion, purpose, or design, with obstinacy; perversely persistent; obstinate; as, pertinacious plotters; a pertinacious beggar.
2.
Resolute; persevering; constant; steady.
Diligence is a steady, constant, and pertinacious study.