Difference between FindInstance and NSolve

Assume that

w[n_] :=Expand[Sum[Binomial[n - k - 1, k]*(-1)^k*A^(n - 2*k - 1), {k, 0, n - 1}]]
f[y_, z_]:=PolynomialRemainder[(w[z] - 1)*(w[y] - 1), (w[i] - 1), A]
h[{y_, z_}] :=Quiet[N[FindRoot[Abs[f[y, z]] - w[i] + 1 == 0, {A, 10000000}]]]
For[i = 153, i < 210, i++,Print[{i}, Max[ParallelMap[h, Subsets[Range[i + 1, 210], {2}]]]]]

According to above code, there are lots of roots.

But, when I try to find them with NSolve, there is no root. Why?

• Actually it gives and a lot of them! One question why you have chosen 10000000 as the initial value? For that initial value it seems that the method is diverging ... – Spawn1701D Apr 10 '13 at 22:45
• @Spawn1701D,Because i want to find the greatest root for all cases. and i guess only where the root? Why does it give? When i try to solve with NSolve, i could not have any solutions. – MATIRMAK Apr 10 '13 at 22:47
• Try just one step, take i = 153 construct the subset Subsets[Range[i + 1, 210], {2}]and take the first pair. if you run h you will get {A -> 5.16816*10^6} substitute this value on Abs[f[y, z]] - w[i] + 1 == 0 and you will get a huge number, in other word for that initial value the method diverges. – Spawn1701D Apr 10 '13 at 22:51

Try this for h:

h[{y_, z_}] :=Quiet[NSolve[Abs[f[y, z]] - w[i] + 1 == 0, A,Reals]]

Also Max won't work try instead this

For[i = 153, i < 210, i++,
Print[{i}, ParallelMap[Sort[h[#]][[-1]]&, Subsets[Range[i + 1, 210], {2}]]]]]

or alternatively,

For[i = 153, i < 210, i++,
Print[{i}, ParallelMap[{A->Max[h[#][[All,All,2]]]}&, Subsets[Range[i + 1, 210], {2}]]]]]

(using Sort is faster)