This paper considers a 2-dissimilar unit warm standby system. The failure rates are constant but the repair rates need not be constant. We shall say that a major breakdown occurs in the system when both unit fail. The system fails if one unit under repair is not repaired within a fixed time measured from the instant at which major breakdown occurs, or if the number of major breakdowns during the mission period exceeds a fixed number. As a special case, this number is allowed to be infinite". The Laplace transform of the reliability and the mean time to system failure (MTSF) are derived. The special case of constant failure and repair rates is treated. We present the asymptotic results which describe the performance of system with short mean repair time. Also the case when allowed down time is negligible, compared to the mission time, is discussed.

As a new method to improve pulse code modulation properties of semiconductor lasers, optical feedback is proposed where a part of the light output from a modulated laser is fed back into itself, with a time delay considerably less than one relaxation oscillation (RO) period. Rate equation approach is applied to investigate the modulation characteristics. Modulation bit rates for small pattern effects depend on the RO period and the current pulse width, and they appear discontinuously. By optical feedback, the RO period increases, and it becomes possible to decrease pattern effects at any given modulation period, if it is larger than the RO period without optical feedback. For current pulses with a short duration, output waveforms are improved by optical feedback through increase in the first RO spike width and elimination of its second spike. In preliminary experiments, using GaAs-Al_XGa_1XAs double heterostructure lasers with a combination of a microlens and a flat mirror, qualitative agreements were obtained between theoretical and experimental results. Improvements in the modulated output waveforms were observed for PCM-RZ pulses at up to 500 Mb/s.

Anomalous re-entrant hysteresis loops and maximum of the coercive force have been found near 130 K in polycrystalline GdIG ferrite. A qualitative explanation of the observed phenomena is given. It is suggested based on the Snoek's model of magnetic after effect that Fe² ion may be responsible for pinning the domain wall.

Several basic properties of minimum-comparison selection algorithms are shown, and some exact values are obtained for V_t(n) for small n, where V_t(n) denotes the number of comparisons necessary to select the t-th largest of n elements. The upper bound for V₅(10) is reduced to 16. Those results improve the table of the best upper bounds known for V_t(n) in the Knuth's book (Sorting and Searching).