2 added 275 characters in body
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Per the comment you left, the code you are using but do not own is licensed under GPLv2 or any later version. That clause may be omitted; the Linux kernel is a famous example of a project that is only licensed as GPLv2. However, it appears that clause is intact in this instance.

Based on the information in the question, it appears that you can license the whole project (including the files in question) under the GPLv3.

Please keep in mind that these are licensing terms. Copyright is still owned by the original author unless reassigned. The fact that you are relicensing the code does not change who actually owns the "butterworth" code.

To remove any potential ambiguity about legal rights, each file should have both a copyright and license notice near the top. You can have a standard GPLv3 notice (the FSF has boilerplate text for this on their web site) but leave the butterworth copyright notice intact.

Per the comment you left, the code you are using but do not own is licensed under GPLv2 or any later version. That clause may be omitted; the Linux kernel is a famous example of a project that is only licensed as GPLv2. However, it appears that clause is intact in this instance.

Based on the information in the question, it appears that you can license the whole project (including the files in question) under the GPLv3.

Please keep in mind that these are licensing terms. Copyright is still owned by the original author unless reassigned. The fact that you are relicensing the code does not change who actually owns the "butterworth" code.

Per the comment you left, the code you are using but do not own is licensed under GPLv2 or any later version. That clause may be omitted; the Linux kernel is a famous example of a project that is only licensed as GPLv2. However, it appears that clause is intact in this instance.

Based on the information in the question, it appears that you can license the whole project (including the files in question) under the GPLv3.

Please keep in mind that these are licensing terms. Copyright is still owned by the original author unless reassigned. The fact that you are relicensing the code does not change who actually owns the "butterworth" code.

To remove any potential ambiguity about legal rights, each file should have both a copyright and license notice near the top. You can have a standard GPLv3 notice (the FSF has boilerplate text for this on their web site) but leave the butterworth copyright notice intact.

1
source | link

Per the comment you left, the code you are using but do not own is licensed under GPLv2 or any later version. That clause may be omitted; the Linux kernel is a famous example of a project that is only licensed as GPLv2. However, it appears that clause is intact in this instance.

Based on the information in the question, it appears that you can license the whole project (including the files in question) under the GPLv3.

Please keep in mind that these are licensing terms. Copyright is still owned by the original author unless reassigned. The fact that you are relicensing the code does not change who actually owns the "butterworth" code.