Invalid patents remain unchallenged

For various reasons, when software patents are considered to be invalid, they're often not proposed for re-examination.

Examples of patents considered invalid but not challenged

• Black Duck's licence compatibility patent
• MPEG video formats - with over 1,000 patents covering these formats, who would bother invalidating one?
• Divine e-commerce patents - the patent holder asked each "violator" to pay less than a challenge would cost

Related pages on ESP Wiki

• Costs of defending are astronomical for developers and SMEs