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Numerical multidimensional infinite series

February 12, 2010

Another new feature in mpmath: support for multidimensional series in nsum. Some very simple examples (finite and infinite summations can be combined in any desired order):

>>> nsum(lambda i,j: 2**(-i-j), [0,inf], [0,inf])
>>> nsum(lambda i,j,k: 2**(-i-j-k), [0,inf], [0,inf], [0,inf])
>>> nsum(lambda i,j,k,l: i/2**j*(i+l)/2**k, [1,2], [0,inf], [1,inf], [2,3])
>>> nsum((lambda i,j,k,l: 1/(2**(i**2+j**2+k**2+l**2))), *([[-inf,inf]]*4))
>>> jtheta(3,0,0.5)**4

One could of course also make nested calls to nsum, but having a direct syntax is much more convenient. Nested evaluation is also usually inefficient for convergence acceleration, so this is not what nsum does internally. Instead, it combines all the infinite summations to a single summation over growing hypercubes. The distinction is very important for conditionally convergent series.

For example, nsum can now directly evaluate the Madelung constant in three dimensions (ignore=True is to ignore the singular term at the origin):

>>> nsum(lambda i,j,k: (-1)**(i+j+k)/(i**2+j**2+k**2)**0.5,
...     [-inf,inf], [-inf,inf], [-inf,inf], ignore=True)

While this evaluation takes several seconds, it is somewhat remarkable that a precise value can be obtained at all. A better way to compute the Madelung constant is using the following rapidly convergent 2D series:

>>> f = lambda i,j: -12*pi*sech(0.5*pi*sqrt((2*i+1)**2+(2*j+1)**2))**2
>>> nsum(f, [0,inf], [0,inf])
>>> mp.dps = 100
>>> nsum(f, [0,inf], [0,inf])

Another nice application is to evaluate Eisenstein series directly from the definition:

>>> tau = 1j
>>> q = qfrom(tau=tau)
>>> nsum(lambda m,n: (m+n*tau)**(-4), [-inf,inf], [-inf,inf], ignore=True)
(3.1512120021539 + 0.0j)
>>> 0.5*sum(jtheta(n,0,q)**8 for n in [2,3,4])*(2*zeta(4))
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