In row 1, the values of a[t] represent na for some c (e.g. RSA 100).
a[t+n] = nb
This is true for all c.
For the value of c, at the same t but in cell (-f,1), the value at a[t] = a(n-1) and a[t+n-1] = b(n-1)
This is the key.
The value of a[t] at -f and e in the first row have the same factor.
The values in each cell that have b as a factor are DIFFERENT, not aligned. They are one element apart in the two cells. In the positive side of the grid, they are n elements apart. In the negative side of the grid, the elements one less elements apart.
One cell has n as a factor at those positions at positive e column, one cells has n-1 as a factor in the negative f column.
This asymmetry can be used to solve the problem.