Although the Ramanujan summation of a divergent series is not a sum in the traditional sense, it has properties that make it mathematically useful in the study of divergent infinite series, for which conventional summation is undefined.
If we take the Euler–Maclaurin summation formula together with the correction rule using Bernoulli numbers, we see that:[clarification needed][further explanation needed] Ramanujan[1] wrote this again for different limits of the integral and the corresponding summation for the case in which p goes to infinity: 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.
we normally recover the usual summation for convergent series.
For functions f(x) with no divergence at x = 1, we obtain: alternatively, applying smoothed sums.
The convergent version of summation for functions with appropriate growth condition is then[citation needed]: To compare, see Abel–Plana formula.
This formula originally appeared in one of Ramanujan's notebooks, without any notation to indicate that it exemplified a novel method of summation.
of 1 − 1 + 1 − ⋯ is: Ramanujan had calculated "sums" of known divergent series.
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
) 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
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.
, Note that this involves (see zeta function regularization) With
, the application of this Ramanujan resummation lends to finite results in the renormalization of quantum field theories.