Dense Locally Testable Codes Cannot Have Constant Rate and Distance

A

q

-query locally testable code (LTC) is an error correcting code that can be tested by a randomized algorithm that reads at most

q

symbols from the given word. An important question is whether there exist LTCs that have the

c

3

property:

c

onstant rate,

c

onstant relative distance, and that can be tested with a

c

onstant number of queries. Such LTCs are sometimes referred to as “asymptotically good”.

We show that

dense

LTCs cannot be

c

3

. The

density

of a tester is roughly the average number of distinct local views in which a coordinate participates. An LTC is

dense

if it has a tester with density

ω

(1).

More precisely, we show that a 3-query locally testable code with a tester of density

ω

(1) cannot be

c

3

. Furthermore, we show that a

q

-locally testable code (

q

 > 3) with a tester of density

ω

(1)

n

q

 − 2

cannot be

c

3

. Our results hold when the tester has the following two properties:

(no weights:) Every

q

-tuple of queries occurs with the same probability.

(‘last-one-fixed’:) In every

q

-query ‘test’ of the tester, the value to any

q

 − 1 of the symbols determines the value of the last symbol. (Linear codes have constraints of this type).

We also show that several natural ways to quantitatively improve our results would already resolve the general

c

3

question, i.e. also for non-dense LTCs.