> Allowing patent on even 'math' should not be an issue as long as the math technique is sufficiently complex that the probablity of someone else independently arriving at that math within the patent term seems to be very low

Your criteria would exclude in excess of 99% of issued software patents. Keep in mind that 20 years is the period of time between the introduction of DNS and the introduction of the iPhone. Twenty years ago Google did not exist and Windows 3.11 was the latest version. Twenty years before that the IBM PC did not exist, Microsoft did not exist, TCP/IP did not exist, AT&T would be The Phone Company (singular) for another decade, the Cold War was in full swing, people were wearing Disco Suits and many of the parents of this year's crop of college graduates had not yet been born.

Unless you meant that the term of a software patent should be on the order of six months to a year, which hardly seems worth the candle.