Is there an English translation of Diophantus's Arithmetica available? It should be in the public domain (obviously), so I'd thought I could find the English text on the web somewhere. Apparently not?
 A: This link says there exists a book, but the cost seems a bit high.
A: Context 
Diophantus' Arithmetica consists of 13 books written originally in Greek circa in ~270 CE.  The original Greek text is lost to us.  The earliest surviving texts are from copies in Arabic transmitted through the Islamic world, before returning to Western Europe in the 1600s via translations from Arabic back into Greek or Latin.
In 1621 CE, Bachet published in Latin the standard text which Fermat read and annotated with his observations.  This makes available 6 of the 13 books.  In 1968, an Arabic text was discovered in Iran containing Books 4-7 of the Arithmetica.  This means 10 of the 13 original books are extent, and the current scholarly view is that Bachet's text has the original Books 1-3, Books 4-7 are from the Arabic text, and the other three books from Bachet are from 8-13, but we don't know which three, and with three books still lost.
Translations to English
Thomas Heath (1910) made an English translation of Bachet's version, but using the text of Tannery (1893, 1895), available freely here:
https://archive.org/details/diophantusofalex00heatiala
It has Books 1-3 & "4-6".
The Tannery text (Greek and Latin) is available here: Vol 1 (1893) and Vol 2 (1895)
The English translation (1982) by Jacques Sesiasno of Books 4-7 can be found here:
https://www.amazon.com/dp/0387906908/
Historical References
The history of Diophantus's Arithmetica here:
www-irma.u-strasbg.fr/~schappa/NSch/Publications_files/1998cBis_Dioph.pdf
A review of Sesiano's translation, with its history, is here:
http://www.jphogendijk.nl/reviews/sesiano.html
A: There is one such translation (freely available), included in the book Diophantus of Alexandria; a study in the history of Greek algebra by Sir Thomas L. Heath (1910).
For some interesting history, user @t.b. recommended (and I fully concur) to look at the paper Diophantus of Alexandria: a text and its history (2005) by Norbert Schappacher (this paper is freely available as well).
