Published online by Cambridge University Press: 17 April 2014
We examine the computable part of the differentiability hierarchy defined byKechris and Woodin. In that hierarchy, the rank of a differentiable function isan ordinal less than ${\omega _1}$ which measures how complex it is to verify differentiabilityfor that function. We show that for each recursive ordinal
$\alpha > 0$, the set of Turing indices of
$C[0,1]$ functions that are differentiable with rank at mostα is
${{\rm{\Pi }}_{2\alpha + 1}}$-complete. This result is expressed in the notation of Ash andKnight.