Finding number pairs where the sum of their divisors are each other

Given a limit \$n\$, find all the pairs of values less than, or equal to n, which have the the property:

pair(a,b)

where a = sum of divisors of b
  and b = sum of divisors of a

I need to speed up my double for-loop but I don't see how to do it in this case.

def diviseurs_propres(n):
    t = [1]
    for i in range(2,n):
        if n%i == 0:
            t.extend([i])
    return t 

def somme_diviseurs_propres(n):
    t = diviseurs_propres(n)
    S = sum(list(t))
    return S

def trouveur(n):
    t = []
    for i in range(0,n+1):
        for p in range(0,n+1):
            if p != i and somme_diviseurs_propres(i) == p and i == somme_diviseurs_propres(p):
             t.extend([p]) and t.extend([i]) 
    return t

Mike is correct that you are repeating the sum multiple times for each value. A simple array with the stored sum will make a huge difference.

On the other hand, you are also doing a huge amount of unnecessary work in here:

def diviseurs_propres(n):
    t = [1]
    for i in range(2,n):
        if n%i == 0:
            t.extend([i])
    return t 

def somme_diviseurs_propres(n):
    t = diviseurs_propres(n)
    S = sum(list(t))
    return S

There is no need to store the actual values in an array, it can be simplified to just:

def somme_diviseurs_propres(n):
    sum = 1
    for i in range(2, n):
        if n % i == 0:
            sum += i
    return sum

With a little bit of math, you can significantly reduce the number of iterations by iterating to the square-root of the value:

def somme_diviseurs_propres(n):
    sum = 1
    root = int(math.ceil(math.sqrt(n)))
    for i in range(2, root):
        if n % i == 0:
            sum += i + (n / i)
    if root > 1 and root * root == n:
        sum += root
    return sum

The way the above code works, is each time you have a value \$a\$ that is a factor of \$n\$, then there must be a value \$n / a\$ that is also an factor.

Thus, for large values of \$n\$, like 1000000, you only need to iterate from 2 to 1000 (exclusive) to sum the factors. Now, because 1000000 is an exact square, the root is an actual integer too, and the last if-statement above adds in the value 1000 to the sum.

In addition to the above, you can also rearrange the double-loops so that the inner loop is limited by the outer loop, and then add the pairs twice to the results. This changes the order of the results, but not the actual content.... Finally, your use of extend should be replaced with append...

def trouveur(n):
    t = []
    for i in range(0, n+1):
        for p in range(0, i):
            if somme_diviseurs_propres(i) == p and i == somme_diviseurs_propres(p):
                t.append(p) and t.append(i) 
                t.append(i) and t.append(p) 
    return t

Now, there's a bug in there, I believe. The code:

t.extend([p]) and t.extend([i])

does not add the second value i. You only add the i on the second time through, when i == p and p == i .... Your code above is equivalent to just:

t.extend([p])

Using a dictionary

As an update, I have considered a second soltuion, that should significantly reduce the amount of iteration in the inner loops, at the expense of lookups in a dictionary.

The dictionary maps the sum-of-factors to the values that have that sum. For example, the sum of factors for 6 and 25 are both 6. (sum of factors of 6 are 3 + 2 + 1, and for 25 it is 5 + 1). The mapping looks like:

6 -> [6, 25]

Using that mapping, we can look for sums that have cross-references... We can loop through just the sums of the factors, and for each sum, in their set of input values, see if those values exist as sums of other values. If they do, then see if the original sum is one of them.... This makes sense if you see an example:

220 -> [284]
284 -> [220]

the top value shows that the sum of factors for 284 is 220, and the sum of factors for 220, is 284.

Taking the first value, we know the sum of factors, and we take all the values that sum to that (in this case, just 284), and check whether 284 happens to be the sum of some other value's factors. Then we check to see whether one of those other values is 220.

def trouveur(n):
    sommeMap = {}
    for i in range(1, n + 1):
        s = somme_diviseurs_propres(i)
        if s in sommeMap:
            sommeMap[s].append(i)
        else:
            sommeMap[s] = [i]

    t = []
    for i,ps in sommeMap.items():
        for p in ps:
            if i < p and p in sommeMap and i in sommeMap[p]:
                t.append(i)
                t.append(p)
    return t

Performance results:

Your initial code, with a limit of 500

Result: [284, 220]

real    0m4.851s
user    0m4.840s
sys     0m0.012s

Using Mike's suggestion of saving away the sums:

def trouveur(n):
    somme = []
    t = []
    for i in range(0,n+1):
        somme.append(somme_diviseurs_propres(i))

    for i in range(0,n+1):
        for p in range(0,n+1):
            if p != i and somme[i] == p and i == somme[p]:
             t.extend([p]) and t.extend([i])
    return t

This produces the times:

Result: [284, 220]

real    0m0.064s
user    0m0.056s
sys     0m0.008s

Including the changes I suggest

• no arrays for the sum of factors

• change the loops

• separate the i and p appends

Code:

```
import math

def somme_diviseurs_propres(n):
sum = 1
root = int(math.ceil(math.sqrt(n)))
for i in range(2, root):
if n % i == 0:
sum += i + (n / i)
if root > 1 and root * root == n:
sum += root
return sum

def trouveur(n):
somme = []
t = []
for i in range(0, n + 1):
somme.append(somme_diviseurs_pro

StackExchange Code Review Q#80740, answer score: 16