[ubuntu] Lenovo laptop won't stay shut off, boots up after shutdown

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http://ubuntuforums.org – I have a ThinkPad S431 running Ubuntu 13.04, and it won't shut off by conventional means. When I choose shutdown, the machine turns off but then promptly turns back on again right away. In order to turn the computer off, I have to reboot, go into the BIOS, then press the power button, after which the machine turns on *again*, then I go into the BIOS again and press the power button, and then it stays off. If I turn the computer on and go into the BIOS right away and press power, it turns off properly. It seems that the problem arises after having booted into Ubuntu. I've already tried ad (Hardware)