Interchange algorithms through a common interface, allowing a client or context to select the policy without embedding every alternative in one implementation. Different strategies solve the same kind of task; the selected algorithm can be varied independently of its caller.

Delegate state-dependent behavior to a separate state object. Transitions replace that object, changing how subsequent requests are handled without spreading state tests through every operation.

Separate operations from a relatively stable hierarchy of element classes. An element's accept operation calls the visitor overload appropriate to its own concrete type, combining virtual dispatch with static overload selection to obtain double dispatch. New operations can be added as visitors; adding an element type generally requires extending the visitor interface and existing visitors.

Keeping sediment acceleration while neglecting water storage and a small gravity-friction residual gives a reduced model for the bed elevation $s$, water speed $u$ and sediment speed $v$. A prescribed water flux $q$ gives $h=q/u$; integrated water momentum gives $s=C-F^2{u}^2/2-q/u$. Around the normalized uniform state $h=u=q=1$, let $(1-v_0{)}^2=\kappa v_0$, $0<v_0<1$. A Fourier mode of wavenumber $k$ has growth-rate real partThus nonuniform modes decay for subcritical Froude number and grow for supercritical Froude number. The spatially constant mode is neutral, and the critical value needs terms neglected in this reduction. In a general uniform state the sign change is at the actual mean-flow Froude number equal to one, rather than at a velocity-scale parameter unrelated to that state.

A credible set is a measurable set of parameter values containing specified posterior probability. At credibility level $1-\alpha$, it satisfies ${\int }_C\pi (\theta \mid x)d\theta =1-\alpha$ for a continuous posterior. A credible interval is a credible set that is an interval. Equal-tailed and highest-density choices add selection rules; the posterior-mass requirement alone does not make the set unique. Its probability statement is conditional on the observed data and prior model, whereas a frequentist confidence interval is interpreted through repeated-sample coverage.

A Banach star-algebra is a complex Banach algebra equipped with a continuous algebra involution. Thus the involution is conjugate-linear, reverses products, and squares to the identity. Some conventions additionally require $\Vert a^{\ast }\Vert =\Vert a\Vert$; this is the isometric convention. In the unital case the involution fixes the unit. The C-star identity is a stronger additional condition. The algebra of continuous complex-valued functions on a compact Hausdorff space, with pointwise operations, complex conjugation and the supremum norm, satisfies both the isometric condition and the C-star identity.

The transitive rank stage $V_{\omega +\omega }$ contains $\omega$ and every ordinal $\omega +n$. The relation mapping a finite ordinal $n$ to the $n$th successor of $\omega$ is internally definable using finite successor chains, whose finite coding overhead remains below this limit rank. Its image would have rank ${\sup }_n(\omega +n+1)=\omega +\omega$, so it is not an element of the stage. This is a failure to collect existing values, rather than a missing individual value.

Membership induction is the schema displayed, with set parameters permitted in $\phi$. The Axiom of foundation proves it by choosing a minimal counterexample in the transitive closure $\mathrm{TC}(\{x\})$ of the singleton containing a counterexample $x$. Conversely, if a nonempty set $a$ had no membership-minimal element, the property $x\notin a$ would be progressive, so induction would force $a$ to be empty. Thus over ZF without Axiom of foundation, this schema implies Axiom of foundation. It also implies that every set lies in the cumulative hierarchy: bound the stage indices of all members of a set by replacement, then place that set in the next power-set stage.

A descending membership sequence is a function on the natural numbers with each next value a member of the preceding value. Foundation excludes it: the range is a nonempty set with no member disjoint from that range.

Every finite ordinal belongs to $V_{\omega }$, so $\omega \in V_{\omega +1}$ but $\omega \notin V_{\omega }$. An internal power set $y$ of $\omega$ would contain $\omega$ itself. Yet $y\in V_{\omega +1}$ implies $y\subseteq V_{\omega }$, making that impossible. This example illustrates why one must check which subsets the structure sees when proving an axiom fails.

If a nonempty set carries a relation in which every point has a successor, the axiom of dependent choice supplies an infinite sequence following that relation. Full choice implies this by selecting a successor at every point and applying the recursion theorem on the natural numbers.

An axiom set is the collection of formulas available as axioms in a specified formal proof system. It may be finite or infinite, or specified by an axiom scheme.

Write the finite cyclic group as $C_n=⟨r⟩$. A group homomorphism is determined by $r\mapsto r^a$; it is a group automorphism exactly when $r^a$ generates $C_n$, equivalently $\gcd (a,n)=1$. Composition multiplies the exponents modulo $n$, proving $\mathrm{Aut}(C_n)\cong (\mathbb{Z}/n\mathbb{Z}{)}^{\times }$. Its group order is therefore Euler's totient function $\varphi (n)$. A group homomorphism $C_2\to \mathrm{Aut}(C_n)$ is determined by a unit $a$ with $a^2\equiv 1(\mathrm{mod}n)$.

Measurement of the mechanical response of the tympanic membrane and middle ear as ear-canal pressure changes.

Liquid-water precipitation reaching the surface. In a catchment water balance its rate is an input measured as water depth per time; only the part not absorbed or otherwise lost becomes rainfall excess feeding overland flow.

An equilibrium has exponential stability when sufficiently small displacements satisfy an estimate $\Vert x(t)-x_{\ast }\Vert \le C{e}^{-\alpha t}\Vert x(0)-x_{\ast }\Vert$ for some $C,\alpha >0$.

The two principal meridians have different powers and are perpendicular. A paraxial pencil forms two perpendicular line foci at distinct depths. A cylindrical lens or toric lens can supply the required meridional correction.^[ref]

An optical irregularity not adequately described by two perpendicular principal meridians of constant power. A simple cylindrical correction may therefore leave residual blur despite correcting a measured average astigmatism.

A simple unital associative algebra is nonzero and has no nonzero proper two-sided ideals. Finite-dimensional complex examples are exactly the full matrix algebras $M_d(\mathbb{C})$, by the Artin–Wedderburn theorem. A simple associative algebra need not be a division algebra: for $d>1$ it has noninvertible nonzero matrices. Any nonzero two-sided ideal of $M_d(\mathbb{C})$ contains a matrix unit, by multiplying a nonzero entry on the left and right by suitable matrix units, and therefore contains all of them.