The constraints on the lowest order terms are as follows.
The higher order terms can be neglected in the limit .
By symmetry the odd order terms in the expansion are zero.
Accuracy can be increased indefinitely by considering higher order terms.
This expression is the leading order term of a Volterra-expansion.
Higher order terms in the series produce novel identities.
For small amplitudes, the higher order terms have little effect.
These can be called the next-to-leading order terms or corrections.
The cubic and higher order terms are assumed to be negligible.
The newer models generally provided higher order terms than their precursors.