where T12ðtÞ;DðtÞ; gðXÞ; f jðXÞ and hjðXÞ are all known functions of their respective arguments and now here we have only one
relaxation function (i ¼ 1 in the expression (14)) as opposed to two in the compressible case (14). In particular, equating (20)
with (19) we must choose