# Solving a large system of equations for the smallest integer values

I'm trying to find the smallest possible non-negative integer solutions to hundreds of fairly large equations systems . An average example would be the following:

expr = {x51 + x61 + y41 + y51 + y61 >= 1,
x51 + x61 + y41 + y51 + y61 == x12 + x13 + x14 + x15 + x16 + y13 + y14 + y15 + y16 == x12 + x23 + x34 + x45 + x56 + y13 + y24 + y35 + y46,
x12 + x62 + y52 + y62 >= 1,
x12 + x62 + y52 + y62 == x23 + x24 + x25 + x26 + y24 + y25 + y26 == x13 + x24 + x35 + x46 + y14 + y25 + y36,
x13 + x23 + y13 + y63 >= 1,
x13 + x23 + y13 + y63 == x34 + x35 + x36 + y35 + y36 == x14 + x25 + x36 + y15 + y26,
x14 + x24 + x34 + y14 + y24 >= 1,
x14 + x24 + x34 + y14 + y24 == x45 + x46 + y41 + y46 == x15 + x26 + y16 + y61,
x15 + x25 + x35 + x45 + y15 + y25 + y35 >= 1,
x15 + x25 + x35 + x45 + y15 + y25 + y35 == x51 + x56 + y51 + y52 == x16 + x61 + y51 + y62,
x16 + x26 + x36 + x46 + x56 + y16 + y26 + y36 + y46 >= 1,
x16 + x26 + x36 + x46 + x56 + y16 + y26 + y36 + y46 == x61 + x62 + y61 + y62 + y63 == x51 + x62 + y41 + y52 + y63,
x51 + x61 + x62 == y13 + y14 + y15 + y16 + y24 + y25 + y26 + y35 + y36 + y46,
y41 + y51 + y52 + y61 + y62 + y63 >= 1 \[Implies] x51 + x61 + x62 >= 1,
{x12, x13, x14, x15, x16, x23, x24, x25, x26, x34, x35, x36, x45, x46, x51, x56, x61, x62, y13, y14, y15, y16, y24, y25, y26, y35, y36, y41, y46, y51, y52, y61, y62, y63} \[Element] NonNegativeIntegers};



Running Solve[] for all the variables does not terminate after several hours. On smaller systems than the one above, I was able to use Normal[Solve[expr,vars]] which gave me a set of rules as the one below:

sols = {{a -> 1 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[8] + C[9],
b -> C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 1 + 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> 3 + C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 2 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> 1 + C[2] + 2 C[5] + C[6] + C[9],
q -> 2 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> C[2] + 2 C[3] + C[4] + C[8]},
{a -> 1 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 1 + 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> 3 + C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 2 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> C[2] + 2 C[5] + C[6] + C[9],
q -> 2 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> 1 + C[2] + 2 C[3] + C[4] + C[8]},
{a -> 1 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> 1 + C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 1 + 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> 1 + C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> 1 + C[2] + 2 C[5] + C[6] + C[9],
q -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> C[2] + 2 C[3] + C[4] + C[8]},
{a -> 1 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> 1 + C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 1 + 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> 1 + C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> C[2] + 2 C[5] + C[6] + C[9],
q -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> 1 + C[2] + 2 C[3] + C[4] + C[8]},
{a -> 2 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> 3 + C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 2 + 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> 2 + C[2] + 2 C[5] + C[6] + C[9],
q -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> C[2] + 2 C[3] + C[4] + C[8]},
{a -> 2 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> 3 + C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 1 + 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 1 + 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> 1 + C[2] + 2 C[5] + C[6] + C[9],
q -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> 1 + C[2] + 2 C[3] + C[4] + C[8]},
{a -> 2 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> 3 + C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 2 + 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> C[1] + C[7],
n -> C[1] + C[7],
o -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> C[2] + 2 C[5] + C[6] + C[9],
q -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> 2 + C[2] + 2 C[3] + C[4] + C[8]},
{a -> 3 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 2 + 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 2 + 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> 2 + C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> 1 + C[1] + C[7],
n -> 1 + C[1] + C[7],
o -> 2 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> C[2] + 2 C[5] + C[6] + C[9],
q -> 2 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> C[2] + 2 C[3] + C[4] + C[8]},
{a -> 3 + 3 C[1] + 2 C[2] + 2 C[3] + C[4] + 2 C[5] + C[6] + 3 C[7] + C[ 8] + C[9],
b -> 1 + C[1] + 3 C[2] + 3 C[3] + C[4] + 3 C[5] + C[6],
c -> 2 + 2 C[1] + C[2] + 2 C[3] + C[4] + 2 C[7] + C[8],
d -> 2 + 2 C[1] + C[2] + 2 C[5] + C[6] + 2 C[7] + C[9],
e -> C[4] + C[6] + 2 C[7] + 3 C[8] + 3 C[9],
m -> 1 + C[1] + C[7],
n -> 1 + C[1] + C[7],
o -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
p -> C[2] + 2 C[5] + C[6] + C[9],
q -> 1 + C[1] + C[2] + C[3] + C[4] + C[5] + C[6] + 2 C[7] + 2 C[8] + 2 C[9],
r -> C[2] + 2 C[3] + C[4] + C[8]}}



I guessed the smallest solutions would be those where all the C[x]'s would be equal to zero, so I created the following array...

czeros = Reap[For[i = 1, i < 60, i++, Sow[C[i] -> 0]]]


... and substituted it back in the set of rules that resulted from the Normal[Solve[]]. I then selected the result that gave the smallest sum of all the variables:

varsum = a + b + c + d + e + m + n + o + p + q + r
list = varsum /. sols /. czeros
min = If[Head[Position[list, Min[list]]] == List, Position[list, Min[list]][[1, 1]], Position[list, Min[list]]]
sols[[min]] /. czeros


Having set the scenario, I have two question:

(1) Does the method above indeed gives the solution with the smallest non-negative sum of the variables?

(2) Is there any alternative method that could be used on systems as large as the first example and that would give results in a reasonable amount of time?

It's my first time using Mathematica, so I'm aware that the code that I'm using might be horribly ineficcient. I have done my best trying to search the documentation for alternatives, but haven´t been able to find any. Thanks in advance for any help.

constraints = {x51 + x61 + y41 + y51 + y61 >= 1,
x51 + x61 + y41 + y51 + y61 ==
x12 + x13 + x14 + x15 + x16 + y13 + y14 + y15 + y16 ==
x12 + x23 + x34 + x45 + x56 + y13 + y24 + y35 + y46,
x12 + x62 + y52 + y62 >= 1,
x12 + x62 + y52 + y62 == x23 + x24 + x25 + x26 + y24 + y25 + y26 ==
x13 + x24 + x35 + x46 + y14 + y25 + y36,
x13 + x23 + y13 + y63 >= 1,
x13 + x23 + y13 + y63 == x34 + x35 + x36 + y35 + y36 ==
x14 + x25 + x36 + y15 + y26, x14 + x24 + x34 + y14 + y24 >= 1,
x14 + x24 + x34 + y14 + y24 == x45 + x46 + y41 + y46 ==
x15 + x26 + y16 + y61,
x15 + x25 + x35 + x45 + y15 + y25 + y35 >= 1,
x15 + x25 + x35 + x45 + y15 + y25 + y35 == x51 + x56 + y51 + y52 ==
x16 + x61 + y51 + y62,
x16 + x26 + x36 + x46 + x56 + y16 + y26 + y36 + y46 >= 1,
x16 + x26 + x36 + x46 + x56 + y16 + y26 + y36 + y46 ==
x61 + x62 + y61 + y62 + y63 == x51 + x62 + y41 + y52 + y63,
x51 + x61 + x62 ==
y13 + y14 + y15 + y16 + y24 + y25 + y26 + y35 + y36 + y46,
y41 + y51 + y52 + y61 + y62 + y63 >= 1 \[Implies]
x51 + x61 + x62 >= 1
};

variables = {x12, x13, x14, x15, x16, x23, x24, x25, x26, x34, x35,
x36, x45, x46, x51, x56, x61, x62, y13, y14, y15, y16, y24, y25,
y26, y35, y36, y41, y46, y51, y52, y61, y62, y63};

{minvarsum, solution} = Minimize[{Total[variables], And @@ constraints},
variables, NonNegativeIntegers]

(** result:
{7, {x12 -> 0, x13 -> 0, x14 -> 0, x15 -> 0, x16 -> 0, x23 -> 0,
x24 -> 0, x25 -> 0, x26 -> 1, x34 -> 0, x35 -> 0, x36 -> 1,
x45 -> 1, x46 -> 0, x51 -> 1, x56 -> 0, x61 -> 0, x62 -> 0,
y13 -> 0, y14 -> 1, y15 -> 0, y16 -> 0, y24 -> 0, y25 -> 0,
y26 -> 0, y35 -> 0, y36 -> 0, y41 -> 0, y46 -> 0, y51 -> 0,
y52 -> 0, y61 -> 0, y62 -> 1, y63 -> 1}}
**)

• Works perfectly! Thank you very much! Commented Nov 7, 2020 at 12:59