  Recently, a permutation-based successive cancellation (PSC) decoding
framework for polar codes attaches much attention. It decodes several permuted
codewords with independent successive cancellation (SC) decoders. Its latency
thus can be reduced to that of SC decoding. However, the PSC framework is
ineffective for permutations falling into the lower-triangular affine (LTA)
automorphism group, as they are invariant under SC decoding. As such, a larger
block lower-triangular affine (BLTA) group that contains SC-variant
permutations was discovered for decreasing polar codes. But it was unknown
whether BLTA equals the complete automorphism group. In this paper, we prove
that BLTA equals the complete automorphisms of decreasing polar codes that can
be formulated as affine trasformations.
