Revision history [back]

I am not sure i understand your question correctly. If you want to iterate over the pairs of integers, you can have a look at multidimensional enumeration:

sage: f = lambda a,b : (a+1)*(b+1)*(a+b+2)
sage: n = 720

sage: for a,b in xmrange([n,n]):
sage:     if f(a,b) == 2*n:
sage:         print a,b      
7 9
9 7

And actually you can be more restrictive and ask a and b to be less than floor(sqrt(2*n)) (instead of n):

sage: bound = floor(sqrt(2*n))
sage: for a,b in xmrange([bound,bound]):
sage:     if f(a,b) == 2*n:
sage:         print a,b      
7 9
9 7

Hence, you get all answers in time $O(n)$ instead of $O(n^2)$. But why not solving directly your equation ? Unfortunately, it seems that the solver is not able to solve the diophantine equation f(a,b) == 2*n directly:

sage: var('a','b')
(a, b)
sage: solve([f(a,b) == 2*n ], a, b)
([a == -1/2*(4*b + sqrt(b^4 + 4*b^3 + 6*b^2 + 5764*b + 5761) + b^2 + 3)/(b + 1), a == -1/2*(4*b - sqrt(b^4 + 4*b^3 + 6*b^2 + 5764*b + 5761) + b^2 + 3)/(b + 1)],
 [1, 1])

But you can still use Sage solver when a is fixed:

sage: var('b')
b
sage: assume(b, 'integer')
sage: assume(b >= 0)

sage: for a in xrange(bound):
sage:     sol = solve([f(a,b) == 2*n],b)
sage:     if len(sol) != 0:
sage:         print a, sol[0].rhs()
sage:         
7 9
9 7

Now, you get an answer in time $0(\sqrt(n))$, which is not satisfactory, but not that bad.

That said, since the solver is quite slow, the multiplicative constant in the complexity is quite huge and the previous method is faster for small n (this is clear for n =6216n=6216 : 42.7ms 42.7ms vs 1.11s). 1.11s). This last method becomes faster only later (this is clear for n =9933n=9933 : 8.79s 8.79s vs 1.41s).

I am not sure i understand your question correctly. If you want to iterate over the pairs of integers, you can have a look at multidimensional enumeration:

sage: f = lambda a,b : (a+1)*(b+1)*(a+b+2)
sage: n = 720

sage: for a,b in xmrange([n,n]):
sage:     if f(a,b) == 2*n:
sage:         print a,b      
7 9
9 7

And actually you can be more restrictive and ask a and b to be less than floor(sqrt(2*n)) (instead of n):

sage: bound = floor(sqrt(2*n))
sage: for a,b in xmrange([bound,bound]):
sage:     if f(a,b) == 2*n:
sage:         print a,b      
7 9
9 7

Hence, you get all answers in time $O(n)$ instead of $O(n^2)$. But why not solving directly your equation ? Unfortunately, it seems that the solver is not able to solve the diophantine equation f(a,b) == 2*n directly:

sage: var('a','b')
(a, b)
sage: solve([f(a,b) == 2*n ], a, b)
([a == -1/2*(4*b + sqrt(b^4 + 4*b^3 + 6*b^2 + 5764*b + 5761) + b^2 + 3)/(b + 1), a == -1/2*(4*b - sqrt(b^4 + 4*b^3 + 6*b^2 + 5764*b + 5761) + b^2 + 3)/(b + 1)],
 [1, 1])

But you can still use Sage solver when a is fixed:

sage: var('b')
b
sage: assume(b, 'integer')
sage: assume(b >= 0)

sage: for a in xrange(bound):
sage:     sol = solve([f(a,b) == 2*n],b)
sage:     if len(sol) != 0:
sage:         print a, sol[0].rhs()
sage:         
7 9
9 7

Now, you get an answer in time $0(\sqrt(n))$, which is not satisfactory, but not that bad.

That said, since the solver is quite slow, the multiplicative constant in the complexity is quite huge and the previous method is faster for small n (this is clear for n=6216 : 42.7ms vs 1.11s). This last method becomes faster only later (this is clear for n=9933n=3065451 : 8.79s94.43s vs 1.41s29.77s).

I am not sure i understand your question correctly. If you want to iterate over the pairs of integers, you can have a look at multidimensional enumeration:

sage: f = lambda a,b : (a+1)*(b+1)*(a+b+2)
sage: n = 720

sage: for a,b in xmrange([n,n]):
sage:     if f(a,b) == 2*n:
sage:         print a,b      
7 9
9 7

And actually you can be more restrictive and ask a and b to be less than floor(sqrt(2*n)) (instead of n):

sage: bound = floor(sqrt(2*n))
sage: for a,b in xmrange([bound,bound]):
sage:     if f(a,b) == 2*n:
sage:         print a,b      
7 9
9 7

Hence, you get all answers in time $O(n)$ instead of $O(n^2)$. But why not solving directly your equation ? Unfortunately, it seems that the solver is not able to solve the diophantine equation f(a,b) == 2*n directly:

sage: var('a','b')
(a, b)
sage: solve([f(a,b) == 2*n ], a, b)
([a == -1/2*(4*b + sqrt(b^4 + 4*b^3 + 6*b^2 + 5764*b + 5761) + b^2 + 3)/(b + 1), a == -1/2*(4*b - sqrt(b^4 + 4*b^3 + 6*b^2 + 5764*b + 5761) + b^2 + 3)/(b + 1)],
 [1, 1])

But you can still use Sage solver when a is fixed:

sage: var('b')
b
sage: assume(b, 'integer')
sage: assume(b >= 0)

sage: for a in xrange(bound):
sage:     sol = solve([f(a,b) == 2*n],b)
sage:     if len(sol) != 0:
sage:         print a, sol[0].rhs()
sage:         
7 9
9 7

Now, you get an answer in time $0(\sqrt(n))$, $O(\sqrt{n})$, which is not satisfactory, completely satisfactory (one could expect a polynomial in $\log(n)$), but not that bad.

That said, since the solver is quite slow, the multiplicative constant in the complexity is quite huge and the previous method is faster for small n (this is clear for n=6216 : 42.7ms vs 1.11s). This The last method becomes faster only later (this is clear for n=3065451 : 94.43s vs 29.77s).