The value (PI)^2/8 is the sum off all of the inverse squares with odd bases only. 1+1/(3^2)+1/(5^2)+1/(7^2)+... It is explained on page 161 of "The Early Mathematics of Leonhard Euler" by C. Edward Sandifer

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The value (PI)^2/8 is the sum off all of the inverse squares with odd bases only. 1+1/(3^2)+1/(5^2)+1/(7^2)+... It is explained on page 161 of "The Early Mathematics of Leonhard Euler" by C. Edward Sandifer