Let $G=⟨S\mid R⟩$ and $H=⟨T\mid U⟩$ be group presentations, and let $\varphi :H\to \mathrm{Aut}(G)$ be a group homomorphism. Choose a word $w_{t,s}(S)$ representing $\varphi (t)(s)$ for each pair of generators. Then the semidirect product has presentationThe cross-relations allow every word to be written in factor order $gh$, and the multiplication rule matches the prescribed action. The natural homomorphisms to and from the semidirect product are inverse on the generators. Finite factor presentations yield a finite group presentation.

For real symmetric $A$, replace sign-vector lifts $x{x}^T$ by all matrices in the elliptope:Every sign vector gives a feasible rank-one matrix with the same objective, so the relaxed maximum is an upper bound. The omitted rank-one condition is substantive: an arbitrary feasible Gram matrix need not come from signs.

In a common finite-volume regularization in quantum field theory, the oscillator contribution is $\Delta M_{\mathrm{osc}}=\frac{1}{2}{\sum }_n({\Omega }_n-{\Omega }_n^{(0)})$, including every discrete and continuum-derived mode and treating translation as a collective coordinate. Physical frequencies are ${\Omega }_n=m{\omega }_n$ when coordinates are measured in units of $m^{-1}$. A mass counterterm is added only afterwards. The vacuum subtraction removes the extensive vacuum energy but can leave an ultraviolet divergence.

For $q=kH$, the even and odd scalar-potential branches have ${\omega }_e^2=(2\pi G\Sigma /H)[q\tanh q-1/(1+\coth q)]$ and ${\omega }_o^2=(2\pi G\Sigma /H)[q\coth q-1/(1+\tanh q)]$. Even potential symmetry moves the surfaces oppositely; odd potential symmetry bends them together. Hydrostatic free-surface pressure gives the positive term and perturbed self-gravity the negative term.

The even surface branch is unstable for $0<kH<q_c$, where $q_c=(1+e^{-2q_c})/2\simeq 0.6392322714$, and stable above this threshold. All positive-wavenumber odd modes are stable because $1-e^{-2q}<2q$. The equilibrium nevertheless has a long-wavelength column-density instability.

On the doubly infinite ladder graph, self-avoiding walks from a fixed graph vertex using only rightward or vertical steps are in bijection with words in $R,V$ containing no consecutive $V$ symbols. Their count is ${\sigma }_n=F_{n+2}$, since ${\sigma }_0=1$, ${\sigma }_1=2$, and ${\sigma }_n={\sigma }_{n-1}+{\sigma }_{n-2}$. Its exponential growth rate is the golden ratio.

In independent bond percolation on a graph, let $c_n$ be the number of $n$-step self-avoiding walks from a fixed graph vertex and ${\kappa }_n$ the number that are open. Then $\mathbb{E}{\kappa }_n=c_n{p}^n$. The Jensen inequality for the concave function $t^{1/n}$ gives $\mathbb{E}({\kappa }_n^{1/n})\le p{c}_n^{1/n}$. No independence between different self-avoiding walk indicators is needed. When $c_n^{1/n}\to \mu$, the limit superior is at most $p\mu$.

Under full semantics for second-order logic, a second-order relation variable of arity $k$ ranges over exactly $\mathcal{P}(D^k)$, the external power set of the appropriate Cartesian product. Thusand universal quantification over $R$ checks every such subset. Here the power set is taken in the ambient mathematical universe, not merely among relations definable by formulas or represented by objects inside $D$.

For $D=\{a,b,c\}$, a unary second-order relation variable has eight possible values, including the three singletons needed by the second-order encoding of three-colorability. The three unary witnesses can range independently over those eight subsets, giving $8^3=512$ candidate triples before the first-order sentence is checked; only six triples pass for the complete graph $K_3$. For $K_4$, there are $16^3=4096$ candidate triples and none passes. Distinguishing the range of candidates from the constraints on valid witnesses clarifies both full semantics for second-order logic and the verifier view of NP.

The appeal of full semantics for second-order logic is its literal reading of assertions about every subset or every relation. A first-order structure fixes the second-order ranges automatically, in contrast to Henkin semantics. That expressive force comes with a limitation: there is no effective, sound deductive system complete for all valid second-order logic sentences under full semantics for second-order logic. This limitation and the change of intended models motivating Henkin semantics are described at the start of ^[ref].

For a domain $V_{\lambda }$, an external functional graph with all its individual ordered pairs in $V_{\lambda }$ is therefore available, even if the whole graph does not belong to $V_{\lambda }$. More generally, a binary second-order relation variable can denote any external subset of $V_{\lambda }\times V_{\lambda }$; its ordered pairs need not themselves be elements of the individual domain. Having endpoints in the domain is what matters. This distinction between internal objects and externally available relations is essential when applying second-order logic to the cumulative hierarchy.

In Henkin semantics, a domain $D$ is accompanied by designated ranges ${\mathcal{H}}_k\subseteq \mathcal{P}(D^k)$ for second-order relation variables of each arity $k$. A formula $\exists R\phi (R)$ searches only ${\mathcal{H}}_k$, and $\forall R\phi (R)$ checks only ${\mathcal{H}}_k$. These ranges can be proper subcollections of the external power set. Keeping $D$ and its input relations fixed therefore need not fix the truth of a second-order logic sentence.

As a toy illustration of restricted quantification, take $D=\{a,b\}$ and unary range ${\mathcal{H}}_1=\{\varnothing ,D\}$. The sentenceis true under full semantics for second-order logic, but false with this selected range, because neither singleton is available. Likewise, on the complete graph $K_3$, the unary range $\{\varnothing ,D\}$ admits no witness for the second-order encoding of three-colorability, even though a usual proper vertex coloring exists. This toy range illustrates the truth definition only: it fails second-order comprehension for the formula $x=a$, and is not a model of a system requiring that comprehension instance.

Admissible Henkin semantics for a specified deductive system must satisfy its axioms, including its chosen second-order comprehension requirements or analogous closure conditions for predicate abstraction. Closure is relative to the permitted formulas and parameters, rather than a blanket property of every selected family. In particular, on a finite domain, allowing logical equality, individual parameters, all finite logical disjunctions, and comprehension for those formulas forces every finite-arity relation to be available: enumerate its tuples and disjoin the formulas naming them. Thus a proper restricted finite range cannot satisfy that entire scheme.

Treating each ${\mathcal{H}}_k$ as an additional sort, with an application relation between its members and $k$ individuals, translates this relational presentation into first-order logic with several sorts. Include axioms saying that predicates with the same members are equal, together with whatever second-order comprehension the system requires. This makes Henkin semantics useful for bringing first-order logic methods to reasoning about predicates; it does not license conclusions about all external relations.

Leon Henkin's Completeness in the Theory of Types, Journal of Symbolic Logic 15 (1950), pp. 81–91 develops general models and proves completeness for his typed deductive system. Its opening footnote 5 explicitly discusses closure under operations associated with formula formation. Henkin later described the discovery process in his ^[ref]; that later account is a historical recollection, distinct from the original publication.

A second-order cone program has a linear objective and affine constraints taking values in products of second-order cones and nonnegative orthants. Norm epigraphs have the form $(t,z)\in \mathcal{Q}$; quadratic epigraphs admit the affine lift $((q+1)/2,(q-1)/2,a)\in {\mathcal{Q}}_3$, equivalent to $q\ge a^2$. Product-cone self-concordant barriers give interior-point methods.

Arithmetical transfinite recursion theory permits arithmetic recursive definitions along coded countable well-orders, together with the usual arithmetic comprehension background. Friedman's finite form of Kruskal's theorem is not provable in this theory.

For a twice continuously differentiable function and $h\ge 0$, the fundamental theorem of calculus givesThe nonnegative triangular weight has integral $h^2$, so its supremum norm estimate directly controls the second modulus of smoothness.

A thermodynamically reversible process can be reversed without a net change of the system and surroundings. Its entropy change equals reversible heat transfer divided by temperature. A reversible adiabatic process conserves entropy; adiabaticity alone does not forbid internal entropy production in an irreversible evolution.

For a nondegenerate induced metric tensor, a totally geodesic submanifold has vanishing second fundamental form; equivalently, every ambient geodesic initially tangent to it remains in it. This extends the notion of a totally geodesic hypersurface to arbitrary codimension.

For the ambient Levi-Civita connection, a normal field $\eta$, and tangent fields $X,Y$, differentiate $⟨\eta ,Y⟩=0$ using metric compatibility. The tangential part of ${\nabla }_{X}Y$ is orthogonal to $\eta$, giving the displayed identity. It controls how an ambient derivative of a normal vector can develop a tangential component, and is the vector-valued version of the shape operator relation.

The Gauss–Codazzi equations relate the intrinsic curvature and connection of an embedded submanifold to its second fundamental form and the ambient curvature. The Gauss equation controls tangential curvature; the Codazzi equation controls the covariant derivative of the second fundamental form. The Gauss formula is the tangential-normal splitting used to derive these relations.

For major radius $a>b>0$ and minor radius $b$, let $R=a+b\cos u$. With the inward normal determined by the usual ordered $(u,v)$ parametrization, the first fundamental form and second fundamental form areReversing the normal reverses $II$. The Gaussian curvature of a ring torus is independent of this orientation.

The retarded coordinate $u=t-r_{\ast }$ puts the radial Schwarzschild metric into $d{s}^2=-fd{u}^2-2dudr+r^{2}d{\Omega }^2$, with $f=1-2M/r$. Outgoing radial null geodesics have constant $u$. These coordinates are regular at the past horizon; an infaller crossing the future horizon has $u\to +\infty$.

For a complex polynomial $p(z)=a{z}^2+bz+c$, assume $|a|>|c|$ and put $D=|a{|}^2-|c{|}^2$ and $E=\overline{a}b-c\overline{b}$. Both roots of a polynomial lie in the closed unit disk if and only if $|E|\le D$. Strict inequality puts both roots in the open disk. Under the strict product assumption $|c/a|<1$, any unit-modulus root is automatically simple.

To prove the strict version, define $p^{\#}(z)=z^2\overline{p(1/\overline{z})}$ and $q=\overline{a}p-c{p}^{\#}=z(Dz+E)$. On the unit circle $|p^{\#}|=|p|$, so the Rouche theorem shows that a stable $p$ and $q$ have the same two interior zeros. Conversely $Dp=aq+c{q}^{\#}$, and the same Rouche theorem implies that two interior zeros of $q$ give two interior zeros of $p$. The nonzero root of $q$ is $-E/D$, proving the criterion. The closed version follows by continuity, replacing $b$ by $(1-\epsilon )b$ in the sufficiency direction. In the necessity direction with a boundary root, replace $p(z)$ by $p(z/r)$ for $r<1$ close to one and let $r\to 1$. The product of the two roots of a polynomial excludes two unit-modulus roots and excludes a repeated unit-modulus root.

Let real power series $f(t)={\sum }_{k\ge 0}{f}_k{t}^k$ and $g(t)={\sum }_{k\ge 0}{g}_k{t}^k$ converge on $[-1,1]$, with $f_k\ge |g_k|$ for every index including zero. If $X⪰0$ has entries there, applying $f$ within two diagonal blocks and $g$ across them preserves positive semidefiniteness. Indeed the coefficient matrix $H_k$ is a bipartite block-constant positive semidefinite matrix. Each $H_k\circ X^{\circ k}$ is positive semidefinite by the Schur product theorem, and their sum converges to the transformed matrix. Closedness of the positive semidefinite cone completes the proof. Convergence at one gives $\sum f_k<\infty$, so coefficient domination ensures absolute convergence of both series throughout the interval. Without the zero-index condition, $f=-1$, $g=0$ and $X=0$ give a counterexample.