 Approximate formulas for some functions of prime numbers by J. Barkley Rosser and Lowell Schoenfeld

Theorem 15, equations (3.41)–(3.42), printed pp. 71–72, give the Rosser–Schoenfeld totient bound. The coefficient 2.5 has an exception at 223092870; 2.50637 covers that case as well. Section 9 contains the proof.