The minimization of makespan in open shop with time lags has been shown NP-hard in the strong sense even for the case of two machines and unit-time operations. The minimization of total completion time however has remained open for that case though it has been shown NP-hard in the strong sense for weighted total completion time or for jobs with release dates. This note shows that the minimization of total completion time for two machines and unit-time operations is NP-hard in the strong sense which answers the long standing open question.

翻译：在开放商店尽可能减少有时滞的黑锅,这从强烈的意义来看是NP硬的,即使在两台机器和单时操作的情况下也是如此,但对于该案来说,将全部完成时间降到最低仍然是开放的,尽管从加权总完成时间或有发布日期的工作的强烈意义来看,已经显示NP硬的。本说明表明,将两台机器和单时操作的总完成时间降到最低,从强烈的意义看,NP硬的,这回答了长期悬而未决的问题。