The quotient criterion is a mathematical convergence criterion for series . It is based on the majorant criterion , that is, a complex series is rated upwards by a simple, here the geometric series . The geometric series converges precisely when the amount of the following terms decreases, i.e. the (constant) quotient of two successive terms is less than 1. If another series decreases at least as quickly from a certain element, i.e. if the quotient is smaller or equal , then this is also convergent. Divergence can also be demonstrated with the quotient criterion. If the quotient is always greater than or equal to 1, the amount of the following terms does not decrease. Since these then do not form a zero sequence , the series is divergent.
The quotient criterion was developed by the mathematician and physicist Jean-Baptiste le Rond d'Alembert , in whose honor this mathematical statement is also called d'Alembert's convergence criterion .
It must not strive towards 1 from below. If, on the other hand, only applies, i.e. the quotient can come as close as desired to 1, the quotient criterion does not provide any information about the convergence or the divergence.
In the case of convergence must of be independent.
Examples
We look at the series and check it for convergence. Using the quotient criterion, we get
.
Hence the series is convergent.
We look at the series and check it for convergence. We obtain
An example of the inapplicability of the quotient criterion is the general harmonic series . It applies
.
For is the general harmonic series divergent, for convergent; however, the quotient criterion cannot distinguish the two cases.
Proof idea
The case of convergence follows with the majorant criterion from the convergence of , a geometric series . The criterion for divergence follows from the fact that the terms can not form a zero sequence because of this.
In contrast to the root criterion , the limes inferior rather than the limes superior must be used for the divergence criterion .
Modified quotient criterion
In addition to the "usual" quotient criterion, there are also the following versions (see also Raabe's criterion ): Be a sequence with genuinely positive terms. If now