Abstract. We consider the problem of counting the number of linear transformation shift registers (TSRs) of a given order over a finite field. We derive explicit formulae for the number of irreducible TSRs of order two. An interesting connection between TSRs and self-reciprocal polynomials is outlined. We use this connection and our results on TSRs to deduce a theorem of Carlitz on the number of self-reciprocal irreducible monic polynomials of a given degree over a finite field.
IntroductionA linear feedback shift register (LFSR) is a mechanism for generating a sequence in a finite field. LFSRs have a plethora of practical applications and are frequently used in generating pseudorandom numbers, fast digital counters and stream ciphers. A generalization of LFSR called word-oriented feedback shift register (σ-LFSR) was considered by Zeng, Han and He [20]. For LFSRs as well as σ-LFSRs, those that are primitive (i.e., for which the corresponding infinite sequence is of maximal possible period) are of particular interest. The following conjecture was proposed in the binary case in [20] and was extended to the q-ary case in [8]:Conjecture 1.1. For positive integers m and n, the number of primitive σ-LFSRs of order n over F q m is given byThe notion of σ-LFSR is essentially equivalent to that of a splitting subspace previously defined by Niederreiter [16]: Given positive integers m, n and α ∈ F q mn , an m-dimensional F q -linear subspace of F q mn is said to be α-splitting if