Hundreds of thousands of small businesses are closing for good. Temporary layoffs at larger companies are becoming permanent. But the country's largest banks, which together serve a majority of Americans through loans, credit cards or deposit services, are not raising an alarm. In their third-quarter earnings reports this week, big banks have said they are generally prepared for a wave of loan defaults they expect in the second half of next year. And their own fortunes are just fine: A trading and investment banking bonanza on Wall Street is helping them stay profitable. A few common themes have emerged from the reports.