Yes, thank you!

I wrote the first version into a general one and it works great! 

[seq([seq([a[i,j],b[((2*N-1)/2-j/2)*j-i+1]],i=N-j..1,-1)], j=1..N-1)];

I spent almost 4hours yesterday afternoon and was not with a clue and you saved my life!

Yes, thank you!

How to deal with this mind me asking also?

[seq([seq([a[i,j],b[??]],i=5-j..1,-1)],j=1..4)]; this has 10 output I guess? How to make b like b[1],b[2]...b[10]?

Thank you very much!