A4tune uses jfftpack:

• jfftpack is a Java version of fftpack. jfftpack is based on Paul N. Swarztraubre's Fortran code and Pekka Janhuen's C code.
• The translation to Java was done by Baoshe Zhang.
• The original fftpack was public domain, so jfftpack is public domain too.

Special thanks to:

• Alberto Nudman for Spanish translations.
• Albert Vitó i Godina and Xavier Conde Rueda for Catalan translations.
• Arseny Plokhikh for Russian translations.
• Eduardo Klosowski for Brazilian Portuguese translations.
• Eugene Sizov and Oleksandr Rizun for Ukrainian translations.
• Francesco Rotondella for Italian translations.
• Fuat Filizkol, Seckin Şahbaz and Tamer Karabulut for Turkish translations.
• Hegyi Zoltán and Stolczenberger Róbert for Hungarian translations.
• Jean-Marc Lartigue for French translations.
• Jędrzej Wojtkowiak, Maciej Michalczyk, Marcin Skulski and Paweł Marciniec for Polish translations.
• Jim Harris for corrections of the English texts.
• Finnish translator who prefers anonymity for Finnish translations.
• Konstantinos Bedoya for Greek translations.
• Mike Ho @HK for Chinese (Traditional) translations.
• Christian Frank, Peter Hahn, Axel Matthia and Mark Paede for German translations.
• Richard Boer for Dutch translations.
• Taeyeong Kim for Korean translations.
• Takayuki Gunge and Tamaki Gunge for Japanese translations.
• Vincent Mozdík for Slovak translations.