Creative works are automatically protected by copyright. No notice is required. Unless you have specific knowledge that a work is in the public domain, you must assume that it is protected.
Having said that, formulas and equations are generally not protected. Graphs may or may not be protected, depending on the degree of creative expression.
Using those equations and graphs as tools to produce independent results would not be a copyright violation.
Still, crediting the original author would, in my opinion, be the correct thing to do.