  In this paper, we present an optimal metric function on average, which leads
to a significantly low decoding computation while maintaining the superiority
of the polarization-adjusted convolutional (PAC) codes' error-correction
performance. With our proposed metric function, the PAC codes' decoding
computation is comparable to the conventional convolutional codes (CC)
sequential decoding. Moreover, simulation results show an improvement in the
low-rate PAC codes' error-correction performance when using our proposed metric
function. We prove that choosing the polarized cutoff rate as the metric
function's bias value reduces the probability of the sequential decoder
advancing in the wrong path exponentially with respect to the wrong path depth.
We also prove that the upper bound of the PAC codes' computation has a Pareto
distribution; our simulation results also verify this. Furthermore, we present
a scaling-bias procedure and a method of choosing threshold spacing for the
search-limited sequential decoding that substantially improves the decoder's
average computation. Our results show that for some codes with a length of 128,
the search-limited PAC codes can achieve an error-correction performance close
to the error-correction performance of the polar codes under successive
cancellation list decoding with a list size of 64 and CRC length of 11 with a
considerably lower computation.
