cert_spec_v1_0.md

MILP certificate file format version 1.0

Preliminaries

Before we describe the mixed-integer linear programming (MILP) certificate file format, we introduce some jargon and conventions used in this document.

Variable indices

Variables of the MILP problem are indexed from $0$ to $n-1$ where $n$ is the total number of variables. Throughout this document, $X_i$ refers to the variable indexed by $i$ regardless of its actual variable name.

Numbers

We refer to two kinds of numbers: indices and values. Indices are integers. Values are rational numbers expressed as finite decimals or fractions.

Constraints

The certificate file includes two kinds of constraints: the constraints of the MILP problem and derived constraints. Constraints have unique constraint names and are assigned indices starting from $0$ according to the order in which they appear in the certificate file.

Each constraint is of the form $a_{i_1}X_{i_1}+\dots+a_{i_p}X_{i_p} \text{sense}\ \beta,$ where sense is $=,\leq$ or $\geq,\beta$ is a rational value, $p\in{1,\dots,n},$ and for $j=1,\dots p,i_j$ is a variable index, $X_{i_j}$ is a variable with index $i_j,$ and $a_{i_j}$ is a rational value.

Bound constraints such as $0\leq\beta\leq 1$ are not treated specially.

Rounding

Suppose that the constraint $a_{i_1}X_{i_1}+\dots+a_{i_p}X_{i_p} \text{sense}\ \beta,$ where sense is $\leq$ or $\geq,$ is such that fpr $j=1,\dots, p,a_{i_j}$ is and integer and is nonzero only if $i_j$ is an integer variable index, then the constraint $a_{i_1}X_{i_1}+\dots+a_{i_p}X_{i_p} \text{sense}\ \beta'$ with $$\beta' = \left { \begin{array}{ll} \lfloor\beta\rfloor & \text{if sense is } \leq \ \lceil\beta\rceil & \text{if sense is } \geq \ \end{array} \right.$$ is said to be obtained from rounding.

Domination of constraints

We use a restricted form of constraint domination. A constraint that is $0\geq \beta$ with $\beta >0$ , $0\leq\beta$ with $\beta<0$, or $0=\beta$ with $\beta\neq 0$ is called an absurdity or falsehood. Such a constraint dominates any other constraint.

The constraint $a^Tx\geq\beta$ or $a^Tx=\beta$ dominates $a'^Tx\geq\beta′$ if $a=a′$ and $\beta\geq\beta'.$

Similarly, the constraint $a^Tx\leq\beta$ or $a^Tx=\beta$ dominates $a′^Tx\leq\beta′$ if $a=a′$ and $\beta\leq\beta'.$

Finally, the constraint $a^Tx=\beta$ dominates $a′^Tx=\beta′$ if $a=a′$ and $\beta=\beta′.$

Suitable linear combinations of constraints

For a constraint named $C$, define $l(C)$ to be the left-hand side of $C$ and define $r(C)$ to be the right-hand side of $C$, and

$$ s(C)=\begin{cases} 1 & \text{if } C \text{ is a } \geq\text{-constraint}\\ 0 & \text{if } C\text{ is a } =\text{-constraint} \\ -1 & \text{if } C\text{ is a } \leq\text{-constraint} \end{cases} $$

For example, for the constraint $C_1: 2X_1+3X_2\geq 1$, we have $l(C_1)=2X_1+3X_2,r(C_1)=1,s(C_1)=1.$ Let $C_1,\dots,C_k$ be constraint names. Let $\lambda_1,\dots,\lambda_k$ be values such that $\lambda_js(C_j)\geq 0$ for all $j=1,…,k,$ or $\lambda_js(C_j)\leq 0$ for all $j=1,…,k.$ Then $\lambda_1\cdot C_1+\dots +\lambda_k \cdot C_k$ is called a suitable linear combination and denotes the constraint

$$ \sum_i\lambda_i l(C_i)=\sum_i\lambda_i r(C_i) \text{ if }\lambda_j s(C_j) = 0 \text{ for all } j=1,\dots,k $$

$$ \sum_i\lambda_i l(C_i)\geq\sum_i\lambda_i r(C_i) \text{ if }\lambda_j s(C_j) \geq 0 \text{ for all } j=1,\dots,k \text{ and } \lambda_q s(C_q)\neq 0 \text{ for some } q\in{1,\dots,k} $$

$$ \sum_i\lambda_i l(C_i)\leq\sum_i\lambda_i r(C_i) \text{ if }\lambda_j s(C_j) \leq 0 \text{ for all } j=1,\dots,k \text{ and } \lambda_q s(C_q)\neq 0 \text{ for some } q\in{1,\dots,k} $$

Constraint format

Specifying a constraint $a_{i_1}X_{i_1}+\cdots+a_{i_p}X_{i_p}\text{ sense }\beta$ in constraint format means listing the details of the constraint in the following order: the constraint name, then the character representing 𝚜𝚎𝚗𝚜𝚎 (E when sense is $=,$ L when sense is $\leq,$ G when sense is $\geq$), then $\beta$, then

• $p$ $i_1$ $a_{i_1}$ $\cdots$ $i_p$ $a_{i_p}$;
• or the keyword OBJ for indicating that $a_{i_1}X_{i_1}+⋯+a_{i_p}X_{i_p}$ is the same as the objective function.

For example, the constraint $−1X_1+6X_2\leq3$ with constraint name C3 specified in constraint format is

C3 L 3  2  1 -1  2 6

If the objective function specified in the certificate file is equal to $−1X_1+6X_2,$ no matter whether to be minimized or maximized, then we may also write the shorter form

C3 L 3 OBJ

Reason format

Every constraint in the DER section specified below is associated with a reason. A reason associated with a constraint with constraint index idx must have one of the following forms:

• { asm }
• { lin $p$ $i_1$ $\lambda_1$ $\cdots$ $i_p$ $\lambda_p$ } where $p$ is a nonnegative integer, $i_1,\dots,i_p$ are distinct indices at least $0$ and less than idx, and $\lambda_1,\dots,\lambda_p$ are rational values such that $\lambda_1\cdot C_{i_1}+\dots+\lambda_p\cdot C_{i_p}$ is a suitable linear combination that dominates the associated constraint; here $C_{i_j}$ denotes the constraint with index $i_j$ for $j=1,\dots,p.$
• { rnd $p$ $i_1$ $\lambda_1$ $\cdots$ $i_p$ $\lambda_p$ } where $p$ is a nonnegative integer, $i_1,\dots,i_p$ are distinct indices at least $0$ and less than idx, and $\lambda_1,\dots,\lambda_p$ are rational values such that $\lambda_1\cdot C_{i_1}+⋯+\lambda_p\cdot C_{i_p}$, is a suitable linear combination that can be rounded and the rounded constraint dominates the associated constraint; again $C_{i_j}$ denotes the constraint with index $i_j$ for $j=1,…,p.$
• { uns $i_1$ $l_1$ $i_2$ $l_2$ } where $i_1, l_1, i_2, l_2$ are indices at least $0$ and less than idx such that the constraints indexed by $i_1$ and $i_2$ both dominate the associated constraint and the constraints indexed by $l_1$ and $l_2$ are, perhaps after reordering, $a_{i_1}X_{i_1}+\cdots a_{i_p}X_{i_p}\leq\beta$ and $a_{i_1}X_{i_1}+\cdots a_{i_p}X_{i_p}\geq\beta +1$ such that $\beta$ is an integer and for $j=1,…,p, a_{i_j}$ is an integer and $X_{i_j}$ is an integer variable.
• { sol }

File format

The initial lines of the file may be comment lines. Each comment line must begin with the character "%". The first line, after the comment lines, must be

VER 1.0

indicating that the certificate file conforms to version 1.0 of the certificate file format.

The remaining content is divided into seven sections and must appear in the following order:

• VAR for the variables,
• INT for integer variables,
• OBJ for the objective function,
• CON for the constraints,
• RTP for the relation to prove,
• SOL for the solutions to check,
• DER for the derivations.

These sections have to be formatted as shown below.

VAR section

The section begins with

VAR n

where $n$ is the number of variables, then followed by the n variable names separated by spaces or line breaks. The variables are assigned indices from $0$ to $n−1$ according to the order in which the variable names are listed. For example, the following specifies two variables x and y:

VAR 2
x y

INT section

The section begins with INT i where $i$ is the number of integer variables, then followed by the $i$ integer variable indices separated by spaces or line breaks. For example, the following specifies our variables x and y as integer variables:

INT 2
0 1

OBJ section

The section begins with OBJ objsense where objsense is the keyword min for minimization or max for maximization, then followed by an integer $k$ and $2k$ numbers $i_1\ c_1\ i_2\ c_2\ \dots\ i_k\ c_k$ separated by spaces or line breaks where for $j=1,…,k, i_j$ is a variable index and $c_k$ is the objective function coefficient for the variable with index $i_j.$

For example, the OBJ section for the problem $$ \begin{array}{ll} \text{min} & x+y \ \text{s.t.} & C_1:4x+y\geq 1 \ & C_2:4x-y\leq 2\ \end{array} $$ could look like the following

OBJ min
2  0 1  1 1

CON section

The section begins with

CON m b where $m$ is the total number of constraints (including bound constraints) and $b$ is the total number of bound constraints, and then followed by $m$ constraints listed in constraint format.

The constraints in this section are assigned indices from $0$ to $m−1$ according to the order in which they are listed. Bound constraints must appear at the beginning of the section before the nonbound constraints.

For example, the CON section for the problem $$\begin{array}{ll} \text{min} & x+y \ \text{s.t.} & C_1:4x+y\geq 1 \ & C_2:4x-y\leq 2\ \end{array} $$ could look like the following

CON 2 0
C1 G 1  2  0 4  1 1
C2 L 2  2  0 4  1 -1

Here, there are no bound constraints but any program that outputs a certificate is free to choose arbitrary names for the bound constraints.

RTP section

The section is either

RTP infeas for indicating infeasibity, or RTP range lb ub where lb specifies the lower bound on the optimal value to be verified and ub specifies the upper bound on the optimal value to be verified. The specified lower bound can be -inf if no actual lower bound is to be verified and the specified upper bound can be inf if no actual upper bound is to be verified. For example, if the RTP section is

RTP range 1 inf

only a lower bound of $1$ is to be verified. In our example, the RTP section looks like this:

RTP range 1 1

SOL section

The section begins with SOL s where s is the number of solutions to be verified for feasibility, and then followed by $S_1 \ \vdots \ S_s$ such that for $j=1,…,s, S_j$ is of the form $\text{name } p\ i_1\ v_1\ \cdots\ i_p\ v_p,$ where name is the solution name, $p$ is a nonnegative integer, and for $r=1,…,p, i_r$ is a variable index and $v_r$ is the solution value for the variable with index $i_r$. All other variables are assumed to have the value zero. At least one of the solutions specified should have objective function value at least the lower bound given in the RTP section in the case of maximization and at most the upper bound given in the RTP section in the case of minimization. For example, to specify the two solutions $(x,y)=(1,2)$ (which is feasible) and $(x,y)=(0,1)$ (which is optimal) where $x$ has variable index 0 and $y$ has variable index 1, the SOL section could look like this:

SOL 2
feas 2  0 1  1 2
opt 1  1 1

DER section

The section begins with DER d where d is the number of derived constraints to be verified, and then followed by $D_1\\vdots\ D_d$ such that $D_1,\dots,D_d$ are assigned constraint indices from $m$ to $m+d−1$ according to the given order and for $j=1,…,d, D_j$ is of the form

constraint reason index

where constraint is the constraint to be verified in constraint format, reason is the reason associated with the constraint in reason format, and index is $−1$ or is an index such that no constraint with a larger index refers to this constraint. Each constraint in this section carries implicitly a set of assumptions deduced as follows:

• If reason is { asm }, then the set of assumptions contains simply the index of the constraint.
• If reason is { lin $p\ i_1\ \lambda_1\ \cdots \ i_p\ \lambda_p$ } or { rnd $p\ i_1\ \lambda_1\ \cdots\ i_p\ \lambda_p$ }, then the set of assumptions is the union of the sets of assumptions of the constraints indexed by $i_1,…,i_p.$
• If reason is { uns $i_1\ l_1\ i_2\ l_2$ }, then the set of assumptions is the union of the sets of assumptions of the constraint indexed by $i_1$ without $l_1$ and of the constraint indexed by $i_2$ without $l_2.$

If the RTP section is RTP infeas, then the last constraint in this section should be an absurdity with an empty set of assumptions. If the RTP section is RTP range lb ub, then the last constraint in this section should be a constraint with an empty set of assumptions that dominates $\text{OBJ}\geq lb$ in the case of minimization or $\text{OBJ}\leq ub$ in the case of maximization where OBJ denotes the objective function.

Remark. The reason for specifying index is to allow a verifier to discard the corresponding constraint from memory once the verifier has completed verifying constraint with index index.

For example, the DER section for the problem above could look like this:

C3 G -1/2  1  1 1   { lin 2  0 1/2  1 -1/2 } 3
C4 G 0     1  1 1   { rnd 1  2 1 } 4
C5 G 1/4   OBJ     { lin 2  0 1/4  3 3/4 } 5
C6 G 1     OBJ     { rnd 1  4 1 } 0

For a small example, we refer to the file paper_eg3.vipr.