Pi with arbitrary precision and mpmath

[Carlos Gouveia]

Hello,

Just for the love of programming, I wrote an arbitrary precision script

to compute Pi using mpmath. I'm not new to mpmath and I like this library

a whole lot. This time I've got puzzling outputs, though.

The algorithm is based on the Borwein brothers' algorithm, which is quite 

simple to program:

With the formulae above I wrote a Python script that spits Pi with an 

arbitrary precision... well, not quite. What I got was a long string

of digits such as the following (only the first five lines shown):

3.14159264621354594737567822448909282684326171875

3.1415926535897935600871733186068013310432434082031

3.1415926535897935600871733186068013310432434082031

3.1415926535897935600871733186068013310432434082031

3.1415926535897935600871733186068013310432434082031

What puzzles me is that the long numerical strings above look like Pi,

but all the digits after the 15th decimal place are wrong. This is an

iterative process that should converge quartically, and the figures

above suggest that Pi has converged after the 2nd iteration, given

the chosen precision. But it converged to the wrong number, and I

wonder why.

Here's the code I've been using:

import mpmath as mp

mp.dps  = 72
mp.pretty = True

one = mp.mpf('1.0')
two = mp.mpf('2.0')
four = mp.mpf('4.0')
six = mp.mpf('6.0')

m = -one/four
tmp = mp.mpf(0)
val = mp.mpf(0)

sqrt2 = mp.sqrt(two)

y = sqrt2 - one
a = six - four*sqrt2

for k in range(0,12):
    tmp = (one - mp.power(y,four))
    tmp = tmp**m
    y = (tmp - one)/(tmp + one)
    a = a*mp.power((one + y),four) - \
        y*mp.power(two,(2*k+3))*(one + y + mp.power(y,2))
    val = one/a
   
    print(mp.nstr(val,50))

Can you spot any mistake in it? The 'algorithmization' is probably

correct, otherwise the script wouldn't yield Pi correct to fifteen

decimal places. It's something wrong I did with mpmath. But what?

Ubuntu 18.04, Python 3.6.8 and mpmath 1.1.0.

Thanks a lot for any light shed on this matter.

C.G.

[Isuru Fernando]

Hi,

You need to do,

mp.mp.dps = 72 instead of mp.dps = 72, otherwise the working precision doesn't change properly.

You can do print(mp.mp) to see the working precision.

Isuru

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[Carlos Gouveia]

Hi Isuru,

That worked, thanks!

C.G.

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