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ANSWER: It depends on what you mean by "commercial software".

1. If by "commercial software" you mean "closed source", then the answer is no, you can't write and publicly distribute such software legally.

If you write a program that genuinely uses the Sage library in a nontrivial way, then that program is a derived work of Sage and must be distributed under the GPL (after all, there is no possible way to run the program without calling many functions in Sage).

When I started Sage, I took PARI -- a GPL'd program -- and started building Sage on top of that. I was forced to GPL Sage because it was a derived work of PARI. It's the same principle at work. Sage is very much a LIBRARY, not just a programming language.

We (Sage developers) also cannot sell or provide you with an exception, because Sage itself depends on many GPL'd programs that we do not own the copyright to.

1. If by "commercial software" you mean software that makes money", then yes, it is possible to build commercial software on top of GPL'd software such as Sage. E.g., SageMathCloud is commercial (it makes money) but is GPL'd. The Linux operating system is also GPL'd but there are companies (like RedHat and Ubuntu) that make money from that software.

ANSWER: It depends on what you mean by "commercial software".

If you write a program that genuinely uses the Sage library in a nontrivial way, then that program is a derived work of Sage and must be distributed under the GPL (after all, there is no possible way to run the program without calling many functions in Sage).

When I started Sage, I took PARI -- a GPL'd program -- and started building Sage on top of that. I was forced to GPL Sage because it was a derived work of PARI. It's the same principle at work. Sage is very much a LIBRARY, not just a programming language.

We (Sage developers) also cannot sell or provide you with an exception, because Sage itself depends on many GPL'd programs that we do not own the copyright to.

ANSWER: It depends on what you mean by "commercial software".

• 1. ONE: If by "commercial software" you mean "closed source", then the answer is no, you can't write and publicly distribute such software legally.

If you write a program that genuinely uses the Sage library in a nontrivial way, then that program is a derived work of Sage and must be distributed under the GPL (after all, there is no possible way to run the program without calling many functions in Sage).

When I started Sage, I took PARI -- a GPL'd program -- and started building Sage on top of that. I was forced to GPL Sage because it was a derived work of PARI. It's the same principle at work. Sage is very much a LIBRARY, not just a programming language.

We (Sage developers) also cannot sell or provide you with an exception, because Sage itself depends on many GPL'd programs that we do not own the copyright to.

• 1. TWO: If by "commercial software" you mean software that makes money", then yes, it is possible to build commercial software on top of GPL'd software such as Sage. E.g., SageMathCloud is commercial (it makes money) but is GPL'd. The Linux operating system is also GPL'd but there are companies (like RedHat and Ubuntu) that make money from that software.