- If the new theory is parallel to the old theory, we may carry the results of
calculations in the old theory over to the new one. It is unnecessary to start
the new theory from scratch.

The results of calculations in the active case can be carried over to the passive case (Compare [Mer2, p.76, l.10] with [Mer2, p.77, l.-11]; compare [Mer2, p.76, l.16] with [Mer2, p.77, (4.118)]). Thus, it is unnecessary to repeat similar calculations for the passive case. The method for defining the boosted wave functions and potentials for the active case [Mer2, p.76, (4.112)] can be used to define their counterparts for the passive case [Mer2, p.77, (4.116)].

- The general formula of the reaction force and that of the reaction moment in [Lan7, p.43, (12.6) & (12.7)]
are very complicated, but for the special cases where the plate is clamped or
supported [Lan7, p.43, Fig.4] the formulas reduce to one or two terms [Lan7, p.44,
(12.9), (12.10) & (12.11)]. This is because some terms in the general formula
vanish and some terms cancel each other. In these cases, it is important to give a
__simple__explanation [Lan7, p.44, l.1-l.2; l.16] of why these terms vanish rather than to give detailed calculations.

- A
__two-term__recursion relation [Lev2, p.136, l.-14-l.-10].

- Model reductions are the crucial steps to understanding the hydrogen atom [Coh,
p.775, l.1-l.20].
- Stationary states of a particle in a
__central__potential [Coh, p.776, l.1]. - Reduction of the two-
__noninteracting__-particle problem to two separate one-particle problems using__separation of variables__[Lev2, p.126, l.11]. - If the potential energy of interaction is a function of the
__relative__coordinates alone, the two-__interacting__-particle problem can be reduced to__two separate__one particle problems [Lev2, p.127, l.1]. - Confinement
of our study in the
__center of mass frame__[Coh, p.791, l.2].

- Stationary states of a particle in a
- Choose the Lorentz condition to
__decouple__the wave equations containing both the electric and magnetic fields [Sad, p.388, l.-6].

- In order to avoid complicated calculations and to obtain the desired
solution
__directly__, we may switch the roles in a previous result with proper modifications.

Example. [Wangs, p.502, l.9-l.12].

- How we reduce calculations when deriving a formula.
- Using general properties instead of special ones.

Example. The derivation of [Lan3, p.67, (26.5)] is simple because Landau uses the general property [Lan3, p.67, l.9].

In contrast, the derivation of [Jack, p.557, (11.143)] is involved because Jackson uses the special property [Lan3, p.557, l.8]. - Reducing to a simpler model so that we can avoid repeating the same calculations used in the simpler model.

Example. When Landau derives [Lan3, p.46, (16.8)], he reduces his calculations from the case of a particle moving in an electromagnetic field to the case of a free particle. Therefore, his derivation is simple. In contrast, when Jackson derives [Jack, p.582, (12.17)], he substitutes [Jack, p.582, (12.16)] into [Jack, p.582, (12.12) & (12.15)] with brutal force. Therefore, Jackson's derivation is less methodical and more complicated.

- Using general properties instead of special ones.