where C is a constant specific to the series and its analytic continuation and the limits on the integral were not specified by Ramanujan, but presumably they were as given above. Comparing both formulae and assuming that R tends to 0 as x tends to infinity, we see that, in a general case, for functions f(x) with no divergence at x = 0:
where Ramanujan assumed By taking we normally recover the usual summation for convergent series. For functions f(x) with no divergence at x = 1, we obtain:
The convergent version of summation for functions with appropriate growth condition is then[citation needed]:
To compare, see Abel–Plana formula.
Ramanujan summation of divergent series
In the following text, indicates "Ramanujan summation". This formula originally appeared in one of Ramanujan's notebooks, without any notation to indicate that it exemplified a novel method of summation.
Ramanujan had calculated "sums" of known divergent series. It is important to mention that the Ramanujan sums are not the sums of the series in the usual sense,[2][3] i.e. the partial sums do not converge to this value, which is denoted by the symbol In particular, the sum of
1 + 2 + 3 + 4 + ⋯
was calculated as:
Extending to positive even powers, this gave:
and for odd powers the approach suggested a relation with the Bernoulli numbers:
It has been proposed to use of C(1) rather than C(0) as the result of Ramanujan's summation, since then it can be assured that one series admits one and only one Ramanujan's summation, defined as the value in 1 of the only solution of the difference equation that verifies the condition .[4]
This demonstration of Ramanujan's summation (denoted as ) does not coincide with the earlier defined Ramanujan's summation, C(0), nor with the summation of convergent series, but it has interesting properties, such as: If R(x) tends to a finite limit when x → 1, then the series is convergent, and we have
In particular we have:
where γ is the
Euler–Mascheroni constant
.
Extension to integrals
Ramanujan resummation can be extended to integrals; for example, using the Euler–Maclaurin summation formula, one can write
which is the natural extension to integrals of the Zeta regularization algorithm.