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2. On Tchebycheff’s Theory of the Totality of the Prime Numbers Comprised within Given Limits;Sylvester, J. J.;Amer. J. Math.,1881
3. E. Landau, Handbuch der Lehre von der Verteilung der Primzahlen, Leipzig, 1909.
4. On the number of primes less than or equal 𝑥;Shapiro, Harold N.;Proc. Amer. Math. Soc.,1950
5. Some Tauberian theorems connected with the prime number theorem;Ingham, A. E.;J. London Math. Soc.,1945