# Min-max theorem

In linear algebra and functional analysis, the min-max theorem, or variational theorem, or Courant–Fischer–Weyl min-max principle, is a result that gives a variational characterization of eigenvalues of compact Hermitian operators on Hilbert spaces. It can be viewed as the starting point of many results of similar nature.
This article first discusses the finite-dimensional case and its applications before considering compact operators on infinite-dimensional Hilbert spaces. We will see that for compact operators, the proof of the main theorem uses essentially the same idea from the finite-dimensional argument.
In the case that the operator is non-Hermitian, the theorem provides an equivalent characterization of the associated singular values. The min-max theorem can be extended to self-adjoint operators that are bounded below.

## Matrices

Let A be a n × n Hermitian matrix. As with many other variational results on eigenvalues, one considers the Rayleigh–Ritz quotient R$_A$ : C$^n$ \ {0} → R defined by
    $R_{A}(x)={\frac {(Ax,x)}{(x,x)}}$
where (⋅, ⋅) denotes the Euclidean inner product on C$^n$. Equivalently, the Rayleigh–Ritz quotient can be replaced by
    $f(x)=(Ax,x),\;\|x\|=1.$
The Rayleigh quotient of an eigenvector $v$ is its associated eigenvalue $\lambda$ because $R_{A}(v)=(\lambda x,x)/(x,x)=\lambda$. For a Hermitian matrix A, the range of the continuous functions R$_A$(x) and f(x) is a compact interval [a, b] of the real line. The maximum b and the minimum a are the largest and smallest eigenvalue of A, respectively. The min-max theorem is a refinement of this fact.

### Min-max theorem

Let A{\textstyle A} be Hermitian on an inner product space V{\textstyle V} with dimension n{\textstyle n}, with spectrum ordered in descending order λ$_1$≥...≥λ$_n${\textstyle \lambda _{1}\geq ...\geq \lambda _{n}}.
Let v$_1$, ..., v$_n${\textstyle v_{1}, ..., v_{n}} be the corresponding unit-length orthogonal eigenvectors.
Reverse the spectrum ordering, so that ξ$_1$=λ$_n$, ..., ξ$_n$=λ$_1${\textstyle \xi _{1}=\lambda _{n}, ..., \xi _{n}=\lambda _{1}}.
(Poincaré’s inequality) — Let M{\textstyle M} be a subspace of V{\textstyle V} with dimension k{\textstyle k}, then there exists unit vectors x, y∈M{\textstyle x, y\in M}, such that
⟨x, Ax⟩≤λ$_k${\textstyle \langle x, Ax\rangle \leq \lambda _{k}}, and ⟨y, Ay⟩≥ξ$_k${\textstyle \langle y, Ay\rangle \geq \xi _{k}}.
Part 2 is a corollary, using −A{\textstyle -A}.
M{\textstyle M} is a k{\textstyle k} dimensional subspace, so if we pick any list of n−k+1{\textstyle n-k+1} vectors, their span N:=span(v$_k$, ...v$_n$){\textstyle N:=span(v_{k}, ...v_{n})} must intersect M{\textstyle M} on at least a single line.
Take unit x∈M∩N{\textstyle x\in M\cap N}. That’s what we need.
    x=∑$_{i=k}$$^n$a$_i$v$_i${\textstyle x=\sum _{i=k}^{n}a_{i}v_{i}}, since x∈N{\textstyle x\in N}.
    Since ∑$_{i=k}$$^n$|a$_i$|$^2$=1{\textstyle \sum _{i=k}^{n}|a_{i}|^{2}=1}, we find ⟨x, Ax⟩=∑$_{i=k}$$^n$|a$_i$|$^2$λ$_i$≤λ$_k${\textstyle \langle x, Ax\rangle =\sum _{i=k}^{n}|a_{i}|^{2}\lambda _{i}\leq \lambda _{k}}.
min-max theorem — ${\begin{aligned}\lambda _{k}&=\max _{\begin{array}{c}{\mathcal {M}}\subset V\\\operatorname {dim} ({\mathcal {M}})=k\end{array}}\min _{\begin{array}{c}x\in {\mathcal {M}}\\\|x\|=1\end{array}}\langle x,Ax\rangle \\&=\min _{\begin{array}{c}{\mathcal {M}}\subset V\\\operatorname {dim} ({\mathcal {M}})=n-k+1\end{array}}\max _{\begin{array}{c}x\in {\mathcal {M}}\\\|x\|=1\end{array}}\langle x,Ax\rangle {\text{. }}\end{aligned}}$
Part 2 is a corollary of part 1, by using −A{\textstyle -A}.
By Poincare’s inequality, λ$_k${\textstyle \lambda _{k}} is an upper bound to the right side.
By setting M=span(v$_1$, ...v$_k$){\textstyle {\mathcal {M}}=span(v_{1}, ...v_{k})}, the upper bound is achieved.
Define the partial trace tr$_V$(A){\textstyle tr_{V}(A)} to be the trace of projection of A{\textstyle A} to V{\textstyle V}. It is equal to ∑$_i$v$_i$$^∗$Av$_i${\textstyle \sum _{i}v_{i}^{*}Av_{i}} given an orthonormal basis of V{\textstyle V}.
Wielandt minimax formula ($^{:44}$) — Let 1≤i$_1$<⋯<i$_k$≤n{\textstyle 1\leq i_{1}<\cdots <i_{k}\leq n} be integers. Define a partial flag to be a nested collection V$_1$⊂⋯⊂V$_k${\textstyle V_{1}\subset \cdots \subset V_{k}} of subspaces of C$^n${\textstyle \mathbb {C} ^{n}} such that dim⁡(V$_j$)=i$_j${\textstyle \operatorname {dim} \left(V_{j}\right)=i_{j}} for all 1≤j≤k{\textstyle 1\leq j\leq k}.
Define the associated Schubert variety X(V$_1$, …, V$_k$){\textstyle X\left(V_{1}, \ldots , V_{k}\right)} to be the collection of all k{\textstyle k} dimensional subspaces W{\textstyle W} such that dim⁡(W∩V$_j$)≥j{\textstyle \operatorname {dim} \left(W\cap V_{j}\right)\geq j}.
$\lambda _{i_{1}}(A)+\cdots +\lambda _{i_{k}}(A)=\sup _{V_{1},\ldots ,V_{k}}\inf _{W\in X\left(V_{1},\ldots ,V_{k}\right)}tr_{W}(A)$
The ≤{\textstyle \leq } case.
Let V$_j$=span(e$_1$, …, e$_{ij}$){\textstyle V_{j}=span(e_{1}, \dots , e_{i_{j}})}, and any W∈X(V$_1$, …, V$_k$){\textstyle W\in X\left(V_{1}, \ldots , V_{k}\right)}, it remains to show that $\lambda _{i_{1}}(A)+\cdots +\lambda _{i_{k}}(A)\leq tr_{W}(A)$
To show this, we construct an orthonormal set of vectors v$_1$, …, v$_k${\textstyle v_{1}, \dots , v_{k}} such that v$_j$∈V$_j$∩W{\textstyle v_{j}\in V_{j}\cap W}. Then tr$_W$(A)≥∑$_j$⟨v$_j$, Av$_j$⟩≥λ$_{ij}$(A){\textstyle tr_{W}(A)\geq \sum _{j}\langle v_{j}, Av_{j}\rangle \geq \lambda _{i_{j}}(A)}
Since dim(V$_1$∩W)≥1{\textstyle dim(V_{1}\cap W)\geq 1}, we pick any unit v$_1$∈V$_1$∩W{\textstyle v_{1}\in V_{1}\cap W}. Next, since dim(V$_2$∩W)≥2{\textstyle dim(V_{2}\cap W)\geq 2}, we pick any unit v$_2$∈(V$_2$∩W){\textstyle v_{2}\in (V_{2}\cap W)} that is perpendicular to v$_1${\textstyle v_{1}}, and so on.
The ≥{\textstyle \geq } case.
For any such sequence of subspaces V$_i${\textstyle V_{i}}, we must find some W∈X(V$_1$, …, V$_k$){\textstyle W\in X\left(V_{1}, \ldots , V_{k}\right)} such that $\lambda _{i_{1}}(A)+\cdots +\lambda _{i_{k}}(A)\geq tr_{W}(A)$
Now we prove this by induction.
The n=1{\textstyle n=1} case is the Courant-Fischer theorem. Assume now n≥2{\textstyle n\geq 2}.
If i$_1$≥2{\textstyle i_{1}\geq 2}, then we can apply induction. Let E=span(e$_{i1}$, …, e$_n$){\textstyle E=span(e_{i_{1}}, \dots , e_{n})}. We construct a partial flag within E{\textstyle E} from the intersection of E{\textstyle E} with V$_1$, …, V$_k${\textstyle V_{1}, \dots , V_{k}}.
We begin by picking a (i$_k$−(i$_1$−1)){\textstyle (i_{k}-(i_{1}-1))}-dimensional subspace W$_k$$^′$⊂E∩V$_{ik}${\textstyle W_{k}'\subset E\cap V_{i_{k}}}, which exists by counting dimensions. This has codimension (i$_1$−1){\textstyle (i_{1}-1)} within V$_{ik}${\textstyle V_{i_{k}}}.
Then we go down by one space, to pick a (i$_{k−1}$−(i$_1$−1)){\textstyle (i_{k-1}-(i_{1}-1))}-dimensional subspace W$_{k−1}$$^′$⊂W$_k$∩V$_{ik−1}${\textstyle W_{k-1}'\subset W_{k}\cap V_{i_{k-1}}}. This still exists. Etc. Now since dim(E)≤n−1{\textstyle dim(E)\leq n-1}, apply the induction hypothesis, there exists some W∈X(W$_1$, …, W$_k$){\textstyle W\in X(W_{1}, \dots , W_{k})} such that $\lambda _{i_{1}-(i_{1}-1)}(A|E)+\cdots +\lambda _{i_{k}-(i_{1}-1)}(A|E)\geq tr_{W}(A)$ Now λ$_{ij−(i1−1)}$(A|E){\textstyle \lambda _{i_{j}-(i_{1}-1)}(A|E)} is the (i$_j$−(i$_1$−1)){\textstyle (i_{j}-(i_{1}-1))}-th eigenvalue of A{\textstyle A} orthogonally projected down to E{\textstyle E}. By Cauchy interlacing theorem, λ$_{ij−(i1−1)}$(A|E)≤λ$_{ij}$(A){\textstyle \lambda _{i_{j}-(i_{1}-1)}(A|E)\leq \lambda _{i_{j}}(A)}. Since X(W$_1$, …, W$_k$)⊂X(V$_1$, …, V$_k$){\textstyle X(W_{1}, \dots , W_{k})\subset X(V_{1}, \dots , V_{k})}, we’re done.
If i$_1$=1{\textstyle i_{1}=1}, then we perform a similar construction. Let E=span(e$_2$, …, e$_n$){\textstyle E=span(e_{2}, \dots , e_{n})}. If V$_k$⊂E{\textstyle V_{k}\subset E}, then we can induct. Otherwise, we construct a partial flag sequence W$_2$, …, W$_k${\textstyle W_{2}, \dots , W_{k}} By induction, there exists some W$^′$∈X(W$_2$, …, W$_k$)⊂X(V$_2$, …, V$_k$){\textstyle W'\in X(W_{2}, \dots , W_{k})\subset X(V_{2}, \dots , V_{k})}, such that $\lambda _{i_{2}-1}(A|E)+\cdots +\lambda _{i_{k}-1}(A|E)\geq tr_{W'}(A)$ thus
$\lambda _{i_{2}}(A)+\cdots +\lambda _{i_{k}}(A)\geq tr_{W'}(A)$ And it remains to find some v{\textstyle v} such that W$^′$⊕v∈X(V$_1$, …, V$_k$){\textstyle W'\oplus v\in X(V_{1}, \dots , V_{k})}.
If V$_1$⊄W$^′${\textstyle V_{1}\not \subset W'}, then any v∈V$_1$∖W$^′${\textstyle v\in V_{1}\setminus W'} would work. Otherwise, if V$_2$⊄W$^′${\textstyle V_{2}\not \subset W'}, then any v∈V$_2$∖W$^′${\textstyle v\in V_{2}\setminus W'} would work, and so on. If none of these work, then it means V$_k$⊂E{\textstyle V_{k}\subset E}, contradiction.
This have some corollaries:$^{:44}$
Extremal partial trace — $\lambda _{1}(A)+\dots +\lambda _{k}(A)=\sup _{\operatorname {dim} (V)=k}tr_{V}(A)$
$\xi _{1}(A)+\dots +\xi _{k}(A)=\inf _{\operatorname {dim} (V)=k}tr_{V}(A)$
Corollary — The sum λ$_1$(A)+⋯+λ$_k$(A){\textstyle \lambda _{1}(A)+\dots +\lambda _{k}(A)} is a convex function, and ξ$_1$(A)+⋯+ξ$_k$(A){\textstyle \xi _{1}(A)+\dots +\xi _{k}(A)} is concave.
(Schur-Horn inequality) $\xi _{1}(A)+\dots +\xi _{k}(A)\leq a_{i_{1},i_{1}}+\dots +a_{i_{k},i_{k}}\leq \lambda _{1}(A)+\dots +\lambda _{k}(A)$ for any subset of indices.
Equivalently, this states that the diagonal vector of A{\textstyle A} is majorized by its eigenspectrum.
Schatten-norm Hölder inequality — Given Hermitian A, B{\textstyle A, B} and Hölder pair 1/p+1/q=1{\textstyle 1/p+1/q=1}, $|\operatorname {tr} (AB)|\leq \|A\|_{S^{p}}\|B\|_{S^{q}}$
WLOG, B{\textstyle B} is diagonalized, then we need to show |∑$_i$B$_{ii}$A$_{ii}$|≤‖A‖$_{Sp}$‖(B$_{ii}$)‖$_{lq}${\textstyle |\sum _{i}B_{ii}A_{ii}|\leq \|A\|_{S^{p}}\|(B_{ii})\|_{l^{q}}}
By the standard Hölder inequality, it suffices to show ‖(A$_{ii}$)‖$_{lp}$≤‖A‖$_{Sp}${\textstyle \|(A_{ii})\|_{l^{p}}\leq \|A\|_{S^{p}}}
By the Schur-Horn inequality, the diagonals of A{\textstyle A} are majorized by the eigenspectrum of A{\textstyle A}, and since the map f(x$_1$, …, x$_n$)=‖x‖$_p${\textstyle f(x_{1}, \dots , x_{n})=\|x\|_{p}} is symmetric and convex, it is Schur-convex.

### Counterexample in the non-Hermitian case

Let N be the nilpotent matrix
    ${\begin{bmatrix}0&1\\0&0\end{bmatrix}}.$
Define the Rayleigh quotient $R_{N}(x)$ exactly as above in the Hermitian case. Then it is easy to see that the only eigenvalue of N is zero, while the maximum value of the Rayleigh quotient is ⁠1/2⁠. That is, the maximum value of the Rayleigh quotient is larger than the maximum eigenvalue.

## Applications
### Min-max principle for singular values

The singular values {σ$_k$} of a square matrix M are the square roots of the eigenvalues of M*M (equivalently MM*). An immediate consequence of the first equality in the min-max theorem is:
    $\sigma _{k}^{\downarrow }=\max _{S:\dim(S)=k}\min _{x\in S,\|x\|=1}(M^{*}Mx,x)^{\frac {1}{2}}=\max _{S:\dim(S)=k}\min _{x\in S,\|x\|=1}\|Mx\|.$
Similarly,
    $\sigma _{k}^{\downarrow }=\min _{S:\dim(S)=n-k+1}\max _{x\in S,\|x\|=1}\|Mx\|.$
Here $\sigma _{k}^{\downarrow }$ denotes the kth entry in the decreasing sequence of the singular values, so that $\sigma _{1}^{\downarrow }\geq \sigma _{2}^{\downarrow }\geq \cdots$.

### Cauchy interlacing theorem

Let A be a symmetric n × n matrix. The m × m matrix B, where m ≤ n, is called a compression of A if there exists an orthogonal projection P onto a subspace of dimension m such that PAP* = B. The Cauchy interlacing theorem states:
    Theorem. If the eigenvalues of A are α$_1$ ≤ ... ≤ α$_n$, and those of B are β$_1$ ≤ ... ≤ β$_j$ ≤ ... ≤ β$_m$, then for all j ≤ m,
       $\alpha _{j}\leq \beta _{j}\leq \alpha _{n-m+j}.$
This can be proven using the min-max principle. Let β$_i$ have corresponding eigenvector b$_i$ and S$_j$ be the j dimensional subspace S$_j$ = span{b$_1$, ..., b$_j$}, then
    $\beta _{j}=\max _{x\in S_{j},\|x\|=1}(Bx,x)=\max _{x\in S_{j},\|x\|=1}(PAP^{*}x,x)\geq \min _{S_{j}}\max _{x\in S_{j},\|x\|=1}(A(P^{*}x),P^{*}x)=\alpha _{j}.$
According to first part of min-max, α$_j$ ≤ β$_j$. On the other hand, if we define S$_{m−j+1}$ = span{b$_j$, ..., b$_m$}, then
    $\beta _{j}=\min _{x\in S_{m-j+1},\|x\|=1}(Bx,x)=\min _{x\in S_{m-j+1},\|x\|=1}(PAP^{*}x,x)=\min _{x\in S_{m-j+1},\|x\|=1}(A(P^{*}x),P^{*}x)\leq \alpha _{n-m+j},$
where the last inequality is given by the second part of min-max.
When n − m = 1, we have α$_j$ ≤ β$_j$ ≤ α$_{j+1}$, hence the name interlacing theorem.

### Lidskii's inequality

Lidskii inequality — If 1≤i$_1$<⋯<i$_k$≤n{\textstyle 1\leq i_{1}<\cdots <i_{k}\leq n} then ${\begin{aligned}&\lambda _{i_{1}}(A+B)+\cdots +\lambda _{i_{k}}(A+B)\\&\quad \leq \lambda _{i_{1}}(A)+\cdots +\lambda _{i_{k}}(A)+\lambda _{1}(B)+\cdots +\lambda _{k}(B)\end{aligned}}$
${\begin{aligned}&\lambda _{i_{1}}(A+B)+\cdots +\lambda _{i_{k}}(A+B)\\&\quad \geq \lambda _{i_{1}}(A)+\cdots +\lambda _{i_{k}}(A)+\xi _{1}(B)+\cdots +\xi _{k}(B)\end{aligned}}$
The second is the negative of the first. The first is by Wielandt minimax.
${\begin{aligned}&\lambda _{i_{1}}(A+B)+\cdots +\lambda _{i_{k}}(A+B)\\=&\sup _{V_{1},\dots ,V_{k}}\inf _{W\in X(V_{1},\dots ,V_{k})}(tr_{W}(A)+tr_{W}(B))\\=&\sup _{V_{1},\dots ,V_{k}}(\inf _{W\in X(V_{1},\dots ,V_{k})}tr_{W}(A)+tr_{W}(B))\\\leq &\sup _{V_{1},\dots ,V_{k}}(\inf _{W\in X(V_{1},\dots ,V_{k})}tr_{W}(A)+(\lambda _{1}(B)+\cdots +\lambda _{k}(B)))\\=&\lambda _{i_{1}}(A)+\cdots +\lambda _{i_{k}}(A)+\lambda _{1}(B)+\cdots +\lambda _{k}(B)\end{aligned}}$
Note that $\sum _{i}\lambda _{i}(A+B)=tr(A+B)=\sum _{i}\lambda _{i}(A)+\lambda _{i}(B)$. In other words, $\lambda (A+B)-\lambda (A)\preceq \lambda (B)$ where $\preceq$ means majorization. By the Schur convexity theorem, we then have
p-Wielandt-Hoffman inequality — ‖λ(A+B)−λ(A)‖$_{ℓp}$≤‖B‖$_{Sp}${\textstyle \|\lambda (A+B)-\lambda (A)\|_{\ell ^{p}}\leq \|B\|_{S^{p}}} where ‖⋅‖$_{Sp}${\textstyle \|\cdot \|_{S^{p}}} stands for the p-Schatten norm.

## Compact operators

Let A be a compact, Hermitian operator on a Hilbert space H. Recall that the spectrum of such an operator (the set of eigenvalues) is a set of real numbers whose only possible cluster point is zero. It is thus convenient to list the positive eigenvalues of A as
    $\cdots \leq \lambda _{k}\leq \cdots \leq \lambda _{1},$
where entries are repeated with multiplicity, as in the matrix case. (To emphasize that the sequence is decreasing, we may write $\lambda _{k}=\lambda _{k}^{\downarrow }$.) When H is infinite-dimensional, the above sequence of eigenvalues is necessarily infinite. We now apply the same reasoning as in the matrix case. Letting S$_k$ ⊂ H be a k dimensional subspace, we can obtain the following theorem.
    Theorem (Min-Max). Let A be a compact, self-adjoint operator on a Hilbert space H, whose positive eigenvalues are listed in decreasing order ... ≤ λ$_k$ ≤ ... ≤ λ$_1$. Then:
       ${\begin{aligned}\max _{S_{k}}\min _{x\in S_{k},\|x\|=1}(Ax,x)&=\lambda _{k}^{\downarrow },\\\min _{S_{k-1}}\max _{x\in S_{k-1}^{\perp },\|x\|=1}(Ax,x)&=\lambda _{k}^{\downarrow }.\end{aligned}}$
A similar pair of equalities hold for negative eigenvalues.
Let S' be the closure of the linear span $S'=\operatorname {span} \{u_{k},u_{k+1},\ldots \}$. The subspace S' has codimension k − 1. By the same dimension count argument as in the matrix case, S' ∩ S$_k$ has positive dimension. So there exists x ∈ S' ∩ S$_k$ with $\|x\|=1$. Since it is an element of S' , such an x necessarily satisfy
    $(Ax,x)\leq \lambda _{k}.$
Therefore, for all S$_k$
    $\inf _{x\in S_{k},\|x\|=1}(Ax,x)\leq \lambda _{k}$
But A is compact, therefore the function f(x) = (Ax, x) is weakly continuous. Furthermore, any bounded set in H is weakly compact. This lets us replace the infimum by minimum:
    $\min _{x\in S_{k},\|x\|=1}(Ax,x)\leq \lambda _{k}.$
So
    $\sup _{S_{k}}\min _{x\in S_{k},\|x\|=1}(Ax,x)\leq \lambda _{k}.$
Because equality is achieved when $S_{k}=\operatorname {span} \{u_{1},\ldots ,u_{k}\}$,
    $\max _{S_{k}}\min _{x\in S_{k},\|x\|=1}(Ax,x)=\lambda _{k}.$
This is the first part of min-max theorem for compact self-adjoint operators.
Analogously, consider now a (k − 1)-dimensional subspace S$_{k−1}$, whose the orthogonal complement is denoted by S$_{k−1}$$^⊥$. If S' = span{u$_1$...u$_k$},
    $S'\cap S_{k-1}^{\perp }\neq {0}.$
So
    $\exists x\in S_{k-1}^{\perp }\,\|x\|=1,(Ax,x)\geq \lambda _{k}.$
This implies
    $\max _{x\in S_{k-1}^{\perp },\|x\|=1}(Ax,x)\geq \lambda _{k}$
where the compactness of A was applied. Index the above by the collection of k-1-dimensional subspaces gives
    $\inf _{S_{k-1}}\max _{x\in S_{k-1}^{\perp },\|x\|=1}(Ax,x)\geq \lambda _{k}.$
Pick S$_{k−1}$ = span{u$_1$, ..., u$_{k−1}$} and we deduce
    $\min _{S_{k-1}}\max _{x\in S_{k-1}^{\perp },\|x\|=1}(Ax,x)=\lambda _{k}.$

## Self-adjoint operators

The min-max theorem also applies to (possibly unbounded) self-adjoint operators. Recall the essential spectrum is the spectrum without isolated eigenvalues of finite multiplicity. Sometimes we have some eigenvalues below the essential spectrum, and we would like to approximate the eigenvalues and eigenfunctions.
    Theorem (Min-Max). Let A be self-adjoint, and let $E_{1}\leq E_{2}\leq E_{3}\leq \cdots$ be the eigenvalues of A below the essential spectrum. Then
$E_{n}=\min _{\psi _{1},\ldots ,\psi _{n}}\max\{\langle \psi ,A\psi \rangle :\psi \in \operatorname {span} (\psi _{1},\ldots ,\psi _{n}),\,\|\psi \|=1\}$.
If we only have N eigenvalues and hence run out of eigenvalues, then we let $E_{n}:=\inf \sigma _{ess}(A)$ (the bottom of the essential spectrum) for n>N, and the above statement holds after replacing min-max with inf-sup.
    Theorem (Max-Min). Let A be self-adjoint, and let $E_{1}\leq E_{2}\leq E_{3}\leq \cdots$ be the eigenvalues of A below the essential spectrum. Then
$E_{n}=\max _{\psi _{1},\ldots ,\psi _{n-1}}\min\{\langle \psi ,A\psi \rangle :\psi \perp \psi _{1},\ldots ,\psi _{n-1},\,\|\psi \|=1\}$.
If we only have N eigenvalues and hence run out of eigenvalues, then we let $E_{n}:=\inf \sigma _{ess}(A)$ (the bottom of the essential spectrum) for n > N, and the above statement holds after replacing max-min with sup-inf.
The proofs use the following results about self-adjoint operators:
    Theorem. Let A be self-adjoint. Then $(A-E)\geq 0$ for $E\in \mathbb {R}$ if and only if $\sigma (A)\subseteq [E,\infty )$.$^{:77}$
    Theorem. If A is self-adjoint, then
$\inf \sigma (A)=\inf _{\psi \in {\mathfrak {D}}(A),\|\psi \|=1}\langle \psi ,A\psi \rangle$
and
$\sup \sigma (A)=\sup _{\psi \in {\mathfrak {D}}(A),\|\psi \|=1}\langle \psi ,A\psi \rangle$.$^{:77}$