Step 3, the sum of fa[] is 63.8306.   π  ≈ h/3 * 63.8306 = 63.8306 / 3 / 7 = 3.039552 ≈ 3.141592653589793… Steps 1 and 2 are MAP  operations. For each implementation the thread and block identification (or local and group identification in OpenCL terminology), are converted into a contiguous range of integers within 0 ..