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// REV 03-19042101 

function pi(n) {
    var v = 0;
    for (i = 1; i <= n; i += 4) {  // increment by 4
        v +=  1 / i - 1 / (i + 2); // add the value of the series
    }
    return 4 * v;                  // apply the factor at last
}