A Tight Composition Theorem for the Randomized Query Complexity of Partial Functions
Abstract
We prove two new results about the randomized query complexity of composed functions. First, we show that the randomized composition conjecture is false: there are families of partial Boolean functions
f
and
g
such that
R(f∘g)≪R(f)R(g)
. In fact, we show that the left hand side can be polynomially smaller than the right hand side (though in our construction, both sides are polylogarithmic in the input size of
f
).
Second, we show that for all
f
and
g
,
R(f∘g)=Ω(noisyR(f)⋅R(g))
, where
noisyR(f)
is a measure describing the cost of computing
f
on noisy oracle inputs. We show that this composition theorem is the strongest possible of its type: for any measure
M(⋅)
satisfying
R(f∘g)=Ω(M(f)R(g))
for all
f
and
g
, it must hold that
noisyR(f)=Ω(M(f))
for all
f
. We also give a clean characterization of the measure
noisyR(f)
: it satisfies
noisyR(f)=Θ(R(f∘gapma
j
n
)/R(gapma
j
n
))
, where
n
is the input size of
f
and
gapma
j
n
is the
n
−
−
√
-gap majority function on
n
bits.