The first three terms, in descending powers of \(x\), in the expansion of

\(\displaystyle \left(2x^2-\frac1{4x}\right)^n\)

can be written in the form

\(256x^{16}+ax^{13}+bx^c\),

where \(n\), \(a\), \(b\) and \(c\) are integers. Find the values of \(n\), \(a\), \(b\) and \(c\).