See the black squares containing 4 numbers each. Each black square has the numbers 1,2,3 and 4 exactly once. Now look at the middle red square. Same property applies. Looks like the regional rule does apply here, unlike the rule given in the Calcdoku prefaces -
"...There is no region rule as in Sudoku..."
...which applies to the 4x4 grid also, mind you, as that has also been mentioned in all the prefaces.
I am wondering if I just uncovered an Easter egg in Brilliant's famous Calcdoku problems. This is because the example 4x4 grid in the prefaces also follows this property -
This is why I would like to explore the other Calcdoku problems and check if I just found a sneaky Easter egg from Brilliant. So if you're a Premium member, please help me out by taking pics of the solved Calcdoku's and sending them to me here.
I have found that Calcdoku 9, which has 4 solution, obeys this rule for each of its solutions.
Sadly, not other Calcdoku's show this property. It was fun trying to find an Easter egg and solving all thosee Calcdoku problems though :)