Print all 3 by 3 sturdy squares

Question

A sturdy square (akin to a magic square) is an arrangement of the integers 1 to N² on an N by N grid such that every 2 by 2 subgrid has the same sum.

For example, for N = 3 one sturdy square is

1 5 3
9 8 7
4 2 6

because the four 2 by 2 subgrids

1 5
9 8

5 3
8 7

9 8
4 2

8 7
2 6

all sum to the same amount, 23:

23 = 1 + 5 + 9 + 8 = 5 + 3 + 8 + 7 = 9 + 8 + 4 + 2 = 8 + 7 + 2 + 6

Now there are sturdy squares for higher values of N and even rectangular versions but your only task in this challenge is to output all possible 3 by 3 sturdy squares. There are exactly 376 distinct 3 by 3 sturdy squares, including those that are reflections or rotations of others, and not all of them have the same sum of 23.

Write a program or function that takes no input but prints or returns a string of all 376 sturdy squares in any order, separated by empty lines, with up to two optional trailing newlines. Each square should consist of three lines of three space separated nonzero decimal digits.

Here is a valid output example:

1 5 3
9 8 7
4 2 6

1 5 6
8 7 3
4 2 9

1 5 6
8 9 3
2 4 7

1 5 7
9 6 3
2 4 8

1 6 2
8 9 7
4 3 5

1 6 2
9 7 8
4 3 5

1 6 3
9 8 7
2 5 4

1 6 7
8 5 2
3 4 9

1 6 7
9 4 3
2 5 8

1 7 2
9 4 8
5 3 6

1 7 2
9 6 8
3 5 4

1 7 4
8 3 5
6 2 9

1 7 4
9 2 6
5 3 8

1 7 6
9 2 4
3 5 8

1 8 2
5 9 4
6 3 7

1 8 3
6 5 4
7 2 9

1 8 3
9 2 7
4 5 6

1 8 4
5 7 2
6 3 9

1 8 4
6 9 3
2 7 5

1 8 4
9 3 6
2 7 5

1 8 6
7 3 2
4 5 9

1 9 2
5 6 4
7 3 8

1 9 2
6 4 5
7 3 8

1 9 2
6 8 5
3 7 4

1 9 2
8 3 7
4 6 5

1 9 3
7 2 5
6 4 8

1 9 3
7 6 5
2 8 4

1 9 4
5 8 2
3 7 6

1 9 4
6 7 3
2 8 5

1 9 4
8 2 5
3 7 6

1 9 5
7 2 3
4 6 8

1 9 5
7 4 3
2 8 6

2 3 5
9 8 6
4 1 7

2 3 6
9 7 5
4 1 8

2 4 3
8 9 7
5 1 6

2 4 3
9 7 8
5 1 6

2 4 6
7 8 3
5 1 9

2 4 7
8 9 3
1 5 6

2 4 8
9 6 3
1 5 7

2 5 3
9 4 8
6 1 7

2 5 4
9 3 7
6 1 8

2 5 4
9 8 7
1 6 3

2 5 7
6 8 1
4 3 9

2 5 7
6 9 1
3 4 8

2 5 8
7 6 1
3 4 9

2 5 8
9 4 3
1 6 7

2 6 1
7 9 8
5 3 4

2 6 1
8 7 9
5 3 4

2 6 3
5 9 4
7 1 8

2 6 4
5 8 3
7 1 9

2 6 7
9 1 4
3 5 8

2 6 8
7 4 1
3 5 9

2 7 1
8 4 9
6 3 5

2 7 1
8 6 9
4 5 3

2 7 3
5 6 4
8 1 9

2 7 3
6 4 5
8 1 9

2 7 3
9 1 8
5 4 6

2 7 5
4 8 1
6 3 9

2 7 5
6 9 3
1 8 4

2 7 5
9 3 6
1 8 4

2 8 1
4 9 5
7 3 6

2 8 4
7 6 5
1 9 3

2 8 5
4 9 1
3 7 6

2 8 5
6 7 3
1 9 4

2 8 6
7 4 3
1 9 5

2 9 1
4 6 5
8 3 7

2 9 1
5 4 6
8 3 7

2 9 1
5 8 6
4 7 3

2 9 1
7 3 8
5 6 4

2 9 3
6 1 5
7 4 8

2 9 4
3 7 1
6 5 8

2 9 4
3 8 1
5 6 7

2 9 5
4 7 1
3 8 6

2 9 5
7 1 4
3 8 6

2 9 6
5 3 1
4 7 8

2 9 6
5 4 1
3 8 7

3 2 5
9 8 7
4 1 6

3 2 6
8 9 5
4 1 7

3 2 7
9 6 5
4 1 8

3 4 2
7 9 8
6 1 5

3 4 2
8 7 9
6 1 5

3 4 5
9 2 7
6 1 8

3 4 8
6 9 1
2 5 7

3 4 9
7 6 1
2 5 8

3 4 9
8 5 2
1 6 7

3 5 1
7 8 9
6 2 4

3 5 2
8 4 9
7 1 6

3 5 4
9 1 8
6 2 7

3 5 4
9 6 8
1 7 2

3 5 8
9 1 4
2 6 7

3 5 8
9 2 4
1 7 6

3 5 9
7 4 1
2 6 8

3 6 1
7 8 9
4 5 2

3 6 2
4 9 5
8 1 7

3 6 8
7 1 2
4 5 9

3 7 2
4 6 5
9 1 8

3 7 2
5 4 6
9 1 8

3 7 2
8 1 9
6 4 5

3 7 4
6 1 5
8 2 9

3 7 4
6 8 5
1 9 2

3 7 6
4 9 1
2 8 5

3 7 6
5 8 2
1 9 4

3 7 6
8 2 5
1 9 4

3 8 1
4 5 6
9 2 7

3 8 1
7 2 9
6 5 4

3 8 4
2 9 1
6 5 7

3 8 6
4 7 1
2 9 5

3 8 6
7 1 4
2 9 5

3 8 7
5 4 1
2 9 6

3 9 1
5 2 7
8 4 6

3 9 1
5 6 7
4 8 2

3 9 2
5 1 6
8 4 7

3 9 4
2 6 1
7 5 8

3 9 4
2 8 1
5 7 6

3 9 6
4 2 1
5 7 8

3 9 6
5 1 2
4 8 7

4 1 6
9 8 7
3 2 5

4 1 7
8 9 5
3 2 6

4 1 7
9 8 6
2 3 5

4 1 8
9 6 5
3 2 7

4 1 8
9 7 5
2 3 6

4 2 6
9 8 7
1 5 3

4 2 7
6 9 3
5 1 8

4 2 7
9 3 6
5 1 8

4 2 8
7 6 3
5 1 9

4 2 9
8 7 3
1 5 6

4 3 5
8 9 7
1 6 2

4 3 5
9 2 8
6 1 7

4 3 5
9 7 8
1 6 2

4 3 7
5 8 2
6 1 9

4 3 7
8 2 5
6 1 9

4 3 7
9 1 6
5 2 8

4 3 9
6 8 1
2 5 7

4 5 2
7 3 9
8 1 6

4 5 2
7 8 9
3 6 1

4 5 3
8 1 9
7 2 6

4 5 3
8 6 9
2 7 1

4 5 6
3 8 1
7 2 9

4 5 6
9 2 7
1 8 3

4 5 9
7 1 2
3 6 8

4 5 9
7 3 2
1 8 6

4 6 2
3 8 5
9 1 7

4 6 5
2 9 1
7 3 8

4 6 5
8 3 7
1 9 2

4 6 8
7 2 3
1 9 5

4 7 1
5 3 8
9 2 6

4 7 1
6 2 9
8 3 5

4 7 3
5 1 6
9 2 8

4 7 3
5 8 6
2 9 1

4 7 5
2 6 1
8 3 9

4 7 8
5 3 1
2 9 6

4 8 1
2 7 5
9 3 6

4 8 1
3 9 6
5 7 2

4 8 1
6 3 9
5 7 2

4 8 2
5 6 7
3 9 1

4 8 3
1 9 2
7 5 6

4 8 6
3 2 1
7 5 9

4 8 7
5 1 2
3 9 6

4 9 1
2 8 5
6 7 3

4 9 1
3 7 6
5 8 2

4 9 1
5 2 8
6 7 3

4 9 2
1 7 3
8 5 6

4 9 2
1 8 3
7 6 5

4 9 3
1 6 2
8 5 7

4 9 3
1 8 2
6 7 5

4 9 5
2 3 1
7 6 8

4 9 5
3 1 2
7 6 8

4 9 6
3 2 1
5 8 7

5 1 6
8 9 7
2 4 3

5 1 6
9 7 8
2 4 3

5 1 8
6 9 3
4 2 7

5 1 8
9 3 6
4 2 7

5 1 9
7 6 3
4 2 8

5 1 9
7 8 3
2 4 6

5 2 3
7 8 9
6 1 4

5 2 8
7 3 4
6 1 9

5 2 8
9 1 6
4 3 7

5 3 2
6 8 9
7 1 4

5 3 4
7 9 8
2 6 1

5 3 4
8 2 9
7 1 6

5 3 4
8 7 9
2 6 1

5 3 6
9 4 8
1 7 2

5 3 8
4 7 1
6 2 9

5 3 8
7 1 4
6 2 9

5 3 8
9 2 6
1 7 4

5 4 3
7 2 9
8 1 6

5 4 6
3 7 2
8 1 9

5 4 6
9 1 8
2 7 3

5 6 4
1 9 2
8 3 7

5 6 4
7 3 8
2 9 1

5 6 7
3 8 1
2 9 4

5 7 2
1 8 4
9 3 6

5 7 2
3 9 6
4 8 1

5 7 2
6 3 9
4 8 1

5 7 4
1 6 2
9 3 8

5 7 6
2 3 1
8 4 9

5 7 6
2 8 1
3 9 4

5 7 6
3 1 2
8 4 9

5 7 8
4 2 1
3 9 6

5 8 2
1 9 4
6 7 3

5 8 2
3 7 6
4 9 1

5 8 7
3 2 1
4 9 6

5 9 1
3 2 7
8 6 4

5 9 1
3 4 7
6 8 2

5 9 2
1 7 4
6 8 3

5 9 2
4 1 7
6 8 3

5 9 4
1 3 2
8 6 7

5 9 4
2 1 3
8 6 7

6 1 4
7 8 9
5 2 3

6 1 5
7 9 8
3 4 2

6 1 5
8 7 9
3 4 2

6 1 7
9 2 8
4 3 5

6 1 7
9 4 8
2 5 3

6 1 8
9 2 7
3 4 5

6 1 8
9 3 7
2 5 4

6 1 9
5 8 2
4 3 7

6 1 9
7 3 4
5 2 8

6 1 9
8 2 5
4 3 7

6 2 3
5 9 8
7 1 4

6 2 4
7 8 9
3 5 1

6 2 7
9 1 8
3 5 4

6 2 8
5 4 3
7 1 9

6 2 9
4 7 1
5 3 8

6 2 9
7 1 4
5 3 8

6 2 9
8 3 5
1 7 4

6 3 2
5 7 9
8 1 4

6 3 5
8 4 9
2 7 1

6 3 7
5 2 4
8 1 9

6 3 7
5 9 4
1 8 2

6 3 9
4 8 1
2 7 5

6 3 9
5 7 2
1 8 4

6 4 2
3 8 7
9 1 5

6 4 5
2 7 3
9 1 8

6 4 5
8 1 9
3 7 2

6 4 8
7 2 5
1 9 3

6 5 1
3 7 8
9 2 4

6 5 1
3 9 8
7 4 2

6 5 4
1 8 3
9 2 7

6 5 4
7 2 9
3 8 1

6 5 7
2 4 1
8 3 9

6 5 7
2 9 1
3 8 4

6 5 8
3 2 1
7 4 9

6 5 8
3 7 1
2 9 4

6 7 1
4 2 9
8 5 3

6 7 3
1 9 4
5 8 2

6 7 3
2 8 5
4 9 1

6 7 3
5 2 8
4 9 1

6 7 5
1 3 2
9 4 8

6 7 5
1 8 2
4 9 3

6 7 5
2 1 3
9 4 8

6 8 1
2 3 7
9 5 4

6 8 2
3 4 7
5 9 1

6 8 3
1 7 4
5 9 2

6 8 3
4 1 7
5 9 2

6 8 4
1 2 3
9 5 7

6 9 2
1 3 5
8 7 4

6 9 2
1 4 5
7 8 3

6 9 3
1 2 4
8 7 5

6 9 3
2 1 5
7 8 4

6 9 4
1 2 3
7 8 5

7 1 4
5 9 8
6 2 3

7 1 4
6 8 9
5 3 2

7 1 6
8 2 9
5 3 4

7 1 6
8 4 9
3 5 2

7 1 8
5 9 4
2 6 3

7 1 9
5 4 3
6 2 8

7 1 9
5 8 3
2 6 4

7 2 3
5 6 9
8 1 4

7 2 4
3 9 6
8 1 5

7 2 4
6 3 9
8 1 5

7 2 6
8 1 9
4 5 3

7 2 9
3 8 1
4 5 6

7 2 9
6 5 4
1 8 3

7 3 4
2 8 5
9 1 6

7 3 4
5 2 8
9 1 6

7 3 4
6 1 9
8 2 5

7 3 6
4 2 5
9 1 8

7 3 6
4 9 5
2 8 1

7 3 8
2 9 1
4 6 5

7 3 8
5 6 4
1 9 2

7 3 8
6 4 5
1 9 2

7 4 2
3 9 8
6 5 1

7 4 8
6 1 5
2 9 3

7 4 9
3 2 1
6 5 8

7 5 1
3 6 9
8 4 2

7 5 2
1 8 6
9 3 4

7 5 2
1 9 6
8 4 3

7 5 6
1 4 2
9 3 8

7 5 6
1 9 2
4 8 3

7 5 8
2 6 1
3 9 4

7 5 9
3 2 1
4 8 6

7 6 1
2 5 8
9 4 3

7 6 1
3 4 9
8 5 2

7 6 2
4 1 9
8 5 3

7 6 5
1 8 3
4 9 2

7 6 8
2 3 1
4 9 5

7 6 8
3 1 2
4 9 5

7 8 3
1 4 5
6 9 2

7 8 4
2 1 5
6 9 3

7 8 5
1 2 3
6 9 4

8 1 4
5 6 9
7 2 3

8 1 4
5 7 9
6 3 2

8 1 5
3 9 6
7 2 4

8 1 5
6 3 9
7 2 4

8 1 6
7 2 9
5 4 3

8 1 6
7 3 9
4 5 2

8 1 7
4 9 5
3 6 2

8 1 9
3 7 2
5 4 6

8 1 9
5 2 4
6 3 7

8 1 9
5 6 4
2 7 3

8 1 9
6 4 5
2 7 3

8 2 4
3 6 7
9 1 5

8 2 5
4 3 7
9 1 6

8 2 5
6 1 9
7 3 4

8 2 6
3 4 5
9 1 7

8 2 9
6 1 5
3 7 4

8 3 5
1 7 4
9 2 6

8 3 5
4 1 7
9 2 6

8 3 5
6 2 9
4 7 1

8 3 7
1 9 2
5 6 4

8 3 7
4 6 5
2 9 1

8 3 7
5 4 6
2 9 1

8 3 9
2 4 1
6 5 7

8 3 9
2 6 1
4 7 5

8 4 2
3 6 9
7 5 1

8 4 3
1 9 6
7 5 2

8 4 6
5 2 7
3 9 1

8 4 7
5 1 6
3 9 2

8 4 9
2 3 1
5 7 6

8 4 9
3 1 2
5 7 6

8 5 2
1 6 7
9 4 3

8 5 2
3 4 9
7 6 1

8 5 3
4 1 9
7 6 2

8 5 3
4 2 9
6 7 1

8 5 6
1 2 3
9 4 7

8 5 6
1 7 3
4 9 2

8 5 7
1 6 2
4 9 3

8 6 2
1 4 7
9 5 3

8 6 3
2 1 7
9 5 4

8 6 4
3 2 7
5 9 1

8 6 7
1 3 2
5 9 4

8 6 7
2 1 3
5 9 4

8 7 4
1 3 5
6 9 2

8 7 5
1 2 4
6 9 3

9 1 5
3 6 7
8 2 4

9 1 5
3 8 7
6 4 2

9 1 6
2 8 5
7 3 4

9 1 6
4 3 7
8 2 5

9 1 6
5 2 8
7 3 4

9 1 7
3 4 5
8 2 6

9 1 7
3 8 5
4 6 2

9 1 8
2 7 3
6 4 5

9 1 8
4 2 5
7 3 6

9 1 8
4 6 5
3 7 2

9 1 8
5 4 6
3 7 2

9 2 4
3 7 8
6 5 1

9 2 6
1 7 4
8 3 5

9 2 6
4 1 7
8 3 5

9 2 6
5 3 8
4 7 1

9 2 7
1 8 3
6 5 4

9 2 7
4 5 6
3 8 1

9 2 8
5 1 6
4 7 3

9 3 4
1 8 6
7 5 2

9 3 6
1 8 4
5 7 2

9 3 6
2 7 5
4 8 1

9 3 8
1 4 2
7 5 6

9 3 8
1 6 2
5 7 4

9 4 3
1 6 7
8 5 2

9 4 3
2 5 8
7 6 1

9 4 7
1 2 3
8 5 6

9 4 8
1 3 2
6 7 5

9 4 8
2 1 3
6 7 5

9 5 3
1 4 7
8 6 2

9 5 4
2 1 7
8 6 3

9 5 4
2 3 7
6 8 1

9 5 7
1 2 3
6 8 4

Your program must produce these same 376 sturdy squares, just not necessarily in this order. The output does not need to be deterministic, i.e. you could output them in different orders on different runs as long as they are all there.

The shortest code in bytes wins.

^{The topic of sturdy squares originated with this chat message of mine which led to a large amount of discussion on their properties and how to generate them. Props to Peter Taylor, feersum, and Sp3000 for continuing the discussion, and especially to El'endia Starman for drafting a corresponding OEIS sequence.}

Answer 1

Pyth, 38 34 33 32 bytes

Vfq2l{sMX2.DR2.:T5b.pS9Vc3NjdH)k

5 bytes saved in formatting by Jakube

1 byte saved by switching to Peter Taylor's substrings of length five, remove the middles approach

Takes about a minute and a half to run on my machine.

How it works at the high level:

• Generate all permutations (.pS9)

• Form length 5 substrings (.:T5)

• Remove the center element of each (.DR2)

• Append a newline to the center element, marking it with a necessarily different sum (X2 ... b)

• Filter for the squares where all such sums are equal (fq2l{)

• Format and print (V ... Vc3NjdH)k)

Answer 2

CJam, 40 38 bytes

A,1>e!3f/{2{2few:::+z}*:|,1=},Ma*Sf*N*

Thanks to @PeterTaylor for golfing off 2 bytes!

This finishes instantly using the Java interpreter. It works using the online interpreter as well, but it requires a little patience. Try it online.

Test run

$ cjam sturdy-squares.cjam | head -n 8
1 5 3
9 8 7
4 2 6

1 5 6
8 7 3
4 2 9

$ cjam sturdy-squares.cjam | tail -n 8

9 5 4
2 3 7
6 8 1

9 5 7
1 2 3
6 8 4
$

How it works

A,1>     e# Push [1 ... 9].
e!       e# Push the array of all permutations of that array.
3f/      e# Split each into rows of length 3.
{        e# Filter; push the permutation, then:
  2{     e#   Do the following twice:
    2few e#     Split each row into overlapping splices of length 2.
         e#       [a b c] -> [[a b] [b c]]
    :::+ e#     Reduce each innermost vector to its sum.
         e#       [[a b] [b c]] -> [a+b b+c]
    z    e#     Transpose rows with columns.
  }*     e#   The result is [[s t] [u v]], the sums of all 2x2 squares.
  :|     e#   Perform set union of the pairs of sums.
  ,1=    e#   Check if the length of the result is 1 (unique sum).
},       e# Keep the array if the result was 1.
{        e# For each kept array:
  Sf*    e#   Join the elements of its rows, separating by spaces.
  ~M     e#   Dump the resulting strings and an empty string on the stack.
}%       e# Collect everything in an array.
N*       e# Join the strings, separating by linefeeds.

Answer 3

Python 3, 169 168 164 bytes

Took the program I used to investigate these sturdy squares/rectangles and golfed it wayyyy down. Golfed off 4 bytes thanks to otakucode.

from itertools import*
r=range(1,10)
for p in permutations(r,6):
 x,y=p[0],p[5];q=p[:5]+(x+p[3]-p[2],y,y+p[1]-x,p[2]+y-x)
 if set(q)==set(r):print('%s %s %s\n'*3%q)

Explanation

Given a partially-filled sturdy square like this,

a b c
d e ?
g ? ?

The remaining three entries are uniquely determined, and are a+d-c, a+b-g, and c+g-a. So I generate all permutations of 0..8 with six elements, calculate the rest, and then check to see whether the set of this is the same as the set of 0..8. If it is, I print out the grid.

---

For reference, here's the original (with comments and extraneous code removed):

from itertools import permutations as P

n = 3
m = 3
permutes = P(range(m*n), m+n)

counter = 0
for p in permutes:
    grid = [p[:n]]
    for i in range(m-1):
        grid.append([p[n+i]]+[-1]*(n-1))
    grid[1][1] = p[-1]

    s = p[0]+p[1]+p[n]+p[-1]

    has = list(p)

    fail = 0
    for y in range(1,m):
        for x in range(1,n):
            if x == y == 1: continue

            r = s-(grid[y-1][x-1] + grid[y-1][x] + grid[y][x-1])

            if r not in has and 0 <= r < m*n:
                grid[y][x] = r
                has.append(r)
            else:
                fail = 1
                break

        if fail: break

    if not fail:
        counter += 1

print(counter)

Answer 4

Mathematica 147 166 155 149 bytes

This generates the permutations of {1,2,3...9} and selects cases for which

(sum of digits at positions {1,2,4,5}) =

(sum of digits at positions {2,3,5,6}) =

(sum of the digits at positions {4,5,7,8}) =

(sum of the digits at positions {5,6,8,9})

f@s_:=Length@Tally[Tr@Extract[s,#]&/@Table[{{0},{1},{3},{4}}+k,{k,{1,2,4,5}}]]>1;
Row[Grid/@(#~Partition~3&/@Select[Permutations@Range@9,f@#&]),"\n"]

Output (a partial look)

---

Length[%]

376

Answer 5

Python 3.5, 135 bytes

from itertools import*
for x in permutations(range(1,10)):eval((("=="+"+x[%s]"*3)*4)[2:]%(*"013125367578",))and print("%d %d %d\n"*3%x)

Directly checks the sum of each square, minus the middle. Most likely still golfable by the "itertools is unnecessary" rule-of-thumb.

Answer 6

CJam (39 37 bytes)

A,1>e!{5ew{2Mtz}2*::+)-!},3f/Ma*Sf*N*

Online demo (warning: may take over a minute to run, triggering "Abort this script?" prompts from the browser).

Works by filtering all possible grids using 5ew to map

[a b c d e f g h i]

to

[[a b c d e]
 [b c d e f]
 [c d e f g]
 [d e f g h]
 [e f g h i]]

and then discarding the middle element and the middle element of each other element to get

[[a b d e]
 [b c e f]
 [d e g h]
 [e f h i]]

which are the four squares.

Answer 7

Python2 327 271 270 263 260 bytes

z,v,s={},3,range(1,10)
while len(z)<376:
 for i in range(8):v=hash(`v`);s[i],s[v%9]=s[v%9],s[i]
 m=map(lambda i:sum(s[i:i+5])-s[i+2],[0,1,3,4]);T=tuple(s)
 if all(x==m[0] for x in m) and not T in z:
  z[T]=1;print '%i %i %i\n'*3 % tuple(s[0:3]+s[3:6]+s[6:9])

------------

This is... not so short but it uses no libraries. This randomly permutes a square, checks it for magicness, prints it, and records it to prevent duplicates. After it has printed 376 unique magic squares, it stops.

I borrowed the Pseudo Random Number Generator from Keith Randall's entry for the golf named " Build a random number generator that passes the Diehard tests "

z,v={},3
def R(x,y):global v;v=hash(`v`);return v
while len(z)<376:
 s=sorted(range(1,10),cmp=R)
 m=[sum(q) for q in map(lambda p:s[p[0]:p[1]+1]+s[p[2]:p[3]+1], [[i,i+1,i+3,i+4] for i in [0,1,3,4]] )]
 if all(x==m[0] for x in m) and not tuple(s) in z.keys():
  z[tuple(s)]=1;print '%i %i %i\n'*3 % tuple(s[0:3]+s[3:6]+s[6:9])

De-golfed

# each magic square is an array of 9 numbers
#
#for example [1 9 3 7 2 5 6 4 8] 
#
#represents the following square
#
#1 9 3
#7 2 5
#6 4 8
#
# to generate a random square with each number represented only once,
# start with [1 2 3 4 5 6 7 8 9] and sort, but use a random comparison
# function so the sorting process becomes instead a random permutation.
# 
# to check each 2x2 subsquare for sums, look at the indexes into the
# array: [[0,1,3,4] = upper left,[1,2,4,5] = upper right, etc.
#
# to keep track of already-printed magic squares, use a dictionary    
# (associative array) where the 9-element array data is the key. 

from random import *
def magic(s):
 quads=[]
 for a,b,c,d in [[0,1,3,4],[1,2,4,5],[3,4,6,7],[4,5,7,8]]:
  quads+=[s[a:b+1]+s[c:d+1]]
 summ=[sum(q) for q in quads]
 same= all(x==summ[0] for x in summ)
 #print quads
 #print 'sum',summ
 #print 'same',same
 return same

magicsquares={}
while len(magicsquares.keys())<376:
        sq = sorted(range(1,10),key=lambda x:random())
        if magic(sq) and not magicsquares.has_key(tuple(sq)):
                magicsquares[tuple(sq)]=1
                print sq[0:3],'\n',sq[3:6],'\n',sq[6:9],'\n'

Answer 8

Ruby 133

a=[]
[*1..9].permutation{|x|[0,1,3,4].map{|i|x[i]+x[i+1]+x[i+3]+x[i+4]}.uniq.size<2&&a<<x.each_slice(3).map{|s|s*' '}*'
'}
$><<a*'

'

Straightforward brute force approach. Test it here.

Answer 9

J, 83 bytes

([:;@,(<LF),.~[:(<@(LF,~":)"1@#~([:*/2=/\[:,2 2+/@,;._3])"2)(3 3)($"1)1+!A.&i.])@9:

This is a function that outputs a string containing the 376 sturdy squares. Uses brute-force, generates all permutations of 1 thru 9, shape each into a 3x3 array, and filters it by checking if the sums of each 2x2 subarray are equal. Completes in half a second.

Usage

   f =: ([:;@,(<LF),.~[:(<@(LF,~":)"1@#~([:*/2=/\[:,2 2+/@,;._3])"2)(3 3)($"1)1+!A.&i.])@9:
   $ f ''  NB. A function has to take something to be invoked,
           NB. but in this case it is not used by the function
   37 {. f ''  NB. Take the first 37 characters
1 5 3
9 8 7
4 2 6

1 5 6
8 7 3
4 2 9

   _38 {. f ''  NB. Take the last 38 characters
9 5 4
2 3 7
6 8 1

9 5 7
1 2 3
6 8 4


   NB. The output string ends with two newlines