Posts: 157 Threads: 30 Joined: Mar 2019 Reputation: **0** Likes received: 1 I am writing a function that approximates the natural logarithm function by using the following formula under iteration: a sub 0= (1+x)/2 b sub 0= sqrt(x) a sub (i+1)= (a sub i + b sub i)/2 b sub (i+1) = sqrt(a sub (i+1) + b sub i) and the approximation to ln(x) is equal to ((x-1)/a sub i) this is the code I came up with but I am not sure I am getting the right results, since when I increase n, the approximation does not get closer to ln(x) (shouldn't it? I'm not sure, if you know, please share), and the graphs I am getting are empty. def applog(n,x):
a0=(1+x)/2
b0=sqrt(x)
for i in range(n):
a0=(a0+b0)/2
b0=sqrt((a0+b0/2)*b0)
return (x-1)/a0
print("the log approximation is: " , ((applog(1,4))))
print("log value-------------------:" , (math.log(4)) )
#of n and get strange result
error=(abs( applog(4,4)- (math.log(4)) ))
print("the error is : ", error)
plt.plot((applog(1,4)) )
plt.plot((math.log(4)))
plt.show() #should plot both funtctions in the same graph
plt.plot(error )
plt.show() # should plot the difference of both functions
Posts: 511 Threads: 1 Joined: Mar 2018 Reputation: **69** Likes received: 69 Apr-14-2019, 10:14 AM (This post was last modified: Apr-14-2019, 10:14 AM by scidam. Edited 1 time in total.) As far as I understood, `sub` stands for sub-index; If so, `sqrt((a0+b0/2)*b0)` doesn't equal to `sqrt(a sub (i+1) + b sub i)` in the formula. So, you have coded another formula than that written above. from math import sqrt, log
def applog(n,x):
a0=(1+x)/2
b0=sqrt(x)
for i in range(n):
a0=(a0+b0)/2
b0 = sqrt(a0 + b0) # This line was changed...
return (x-1)/a0
for j in range(2, 45):
error=(abs( applog(j,4)- (log(4))))
print(f"{j}:{error}") Output: 2:0.0053156365995163934
3:0.039090279331435385
4:0.061621145018602386
5:0.07728516949056563
6:0.08826319537752325
7:0.09595463102864743
8:0.10133370381925122
9:0.10508932614514
10:0.10770813826749515
11:0.10953258138230271
12:0.11080279652612712
13:0.11168674981804516
14:0.1123017079623232
15:0.11272943559557191
16:0.11302689201201455
17:0.11323373160573724
18:0.11337754927182253
19:0.11347754206638672
20:0.11354706207495191
21:0.11359539468519486
....
So, iterative process doesn't converge to the expected solution... The iterative formula seems to be wrong... mcgrim likes this post Posts: 157 Threads: 30 Joined: Mar 2019 Reputation: **0** Likes received: 1 Thanks, I understand what you are saying, I came up with a correction to my code, however my graphs are still empty. I'd like to know why so and what I should do in order to solve this issue. def applog(n,x):
a0=(1+x)/2
b0=sqrt(x)
for i in range(n):
a0=(a0+b0)/2
b0=sqrt((a0)*b0)
return (x-1)/a0
print("the log approximation is: " , ((applog(100,4))))
print("log value-------------------:" , (np.log(4)) )
error=(abs( applog(100,4)- (np.log(4)) ) )
print("the error is : ", error)
plt.show()
plt.plot((applog(100,4)) )
plt.plot(np.log(4))
plt.show() #should plot both funtctions in the same graph
plt.plot(error )
plt.show() # should plot the difference of both functions
Posts: 511 Threads: 1 Joined: Mar 2018 Reputation: **69** Likes received: 69 Ok, now iterative process is converged! Take a look at the following piece of code: from math import sqrt, log
import numpy as np
import matplotlib.pyplot as plt
@np.vectorize
def applog(n, x):
a = (1 + x) / 2
b = sqrt(x)
for i in range(n):
a = (a + b) / 2
b = sqrt(a * b)
return (x - 1) / a
allowed_n = 2, 3, 4, 5, 6
colors = 'crgby'
fig = plt.figure()
ax = fig.add_subplot(111)
x = np.linspace(10, 500, 100)
for n, c in zip(allowed_n, colors):
ax.plot(x, applog(n, x), color=c, label=n)
plt.legend(title='N')
plt.show() Posts: 157 Threads: 30 Joined: Mar 2019 Reputation: **0** Likes received: 1 Thanks for your help. After looking at your code, I tried to come up with the same result even though I mainly used code I am already familiar with. I am trying to plot in the same graph, the function for different values of the range, that's why I turned it into a list, however, I am still getting only one value. Can you please tell me how my code should be corrected? def applog(a,x):
a0=(1+x)/2
b0=sqrt(x)
a=[]
for i in range(n):
a0=(a0+b0)/2
b0=sqrt((a0)*b0)
a.append(n)
return np.array((x-1)/a0)
print("the log approximation is: " , ((applog(4,4))))
print("log value-------------------:" , (np.log(4)) )
error=(abs( applog(4,4)- (np.log(4)) ) )
print("the error is : ", error)
x = np.linspace(10, 500, 100)
plt.plot(x, (applog(4,x)),color='blue', label='approx ln(x)' )
plt.plot(x, np.log(x), color='red', label='ln(x)')
plt.legend(loc='upper left')
plt.show()
plt.plot(x,abs(np.log(x)-applog(4,x)) )
plt.show()
Posts: 511 Threads: 1 Joined: Mar 2018 Reputation: **69** Likes received: 69 You have two issues with your code: 1) `def applog(a,x):` should be `def applog(n,x):` 2)You need to add the following lines to your code: y = [applog(4, _) for _ in x]
plt.plot(x, y,color='blue', label='approx ln(x)' ) In my code, I used `np.vectorize` decorator to be able to pass vectors to the `applog` function. E.g. after applying `np.vectorize` we are able to pass vectors to applog: e.g. `applog(4, [1,2,3,4,5])` is allowed now. So, if don't want to use `np.vectorize` you need to precompute y-values corresponding to x-values. This is why I add a line `y = ....` . Posts: 157 Threads: 30 Joined: Mar 2019 Reputation: **0** Likes received: 1 thank you, I solved it. Now there is a continuation which makes me feel a bit lost. the new a sub n is called d sub (0,n) and d sub (k,n)=( d sub(k-1, n) -2^(-2k) d sub(k-1, n-1) )/(1-2^(-2k)) where n=0,1,2.... and k=1,2,3...n The function should return (x-1)/d sub(n,n). I really have no idea how to even start this. I hope to get some help as this is very confusing. Posts: 511 Threads: 1 Joined: Mar 2018 Reputation: **69** Likes received: 69 This problem seems to me unclear and confuses me too. I would like to see its solution... Posts: 157 Threads: 30 Joined: Mar 2019 Reputation: **0** Likes received: 1 Apr-18-2019, 07:20 AM (This post was last modified: Apr-18-2019, 07:31 AM by mcgrim. Edited 1 time in total.) The solution is similar to the previous task, it just reaches convergence faster. I am here including a link where this formula is written (2.4). Let me know if this is enough or you need more info. https://www.ams.org/journals/mcom/1972-2...7438-2.pdf Posts: 157 Threads: 30 Joined: Mar 2019 Reputation: **0** Likes received: 1 |