What goes down must come up, right? The Dow Jones has risen 6.3 percent so far today after central banks injected billions of dollars of liquidity into the financial system over the weekend. Markets abroad did even better: The German and French exchanges were up over 11 percent, while the British exchange was up 8.2 percent. The strength of today’s gains will be tested tomorrow, however, when credit markets reopen after the Columbus Day holiday. If they remain frozen, stocks will probably tumble once again.