This is such an inspiring true story that it is quite surprising that Sophie's Diary appears to be the first time anyone has tried to fill in the personal details in a fictionalized first person account. I suspect that this book, which contains detailed descriptions of the events of the French revolution as well as a good deal of mathematics, was written specifically to motivate mathematically talented girls to study and pursue careers in mathematics. (The description of the author indicates that she won a mathematics contest at age six and went on to study aerospace engineering.)

You can read more about this work at other websites such as:

• the official website of the book (including excerpts and biographical information)]
• a review from the MAA
• Google Books (where you can read quite a few pages of the book for free)
• and, of course, Amazon.com

I do hope that this book can inspire some future mathematicians, especially girls like Sophie who have to battle sexism to follow their avocation. Certainly, the emotional aspects of the true story combined with the first person account seem like a winning combination. As Judy Holdener says in her MAA review: "The author is successful in capturing both the passion and the frustration commonly felt by mathematicians."

However, I am a bit worried by possible confusion caused by some ambiguous or misleading mathematical statements. For instance, when discussing the number π, Sophie writes:

Perhaps it is arguing semantics to wonder about her description "the exact value is still unknown". (I would say the exact value is known, but cannot be written in finite decimal form.) But, her suggestion that it is unknown how many digits are needed is a bit more troubling. It was known since 1761 (before the date on Sophie's diary entry) that π is an irrational number...and in fact she says so herself in the book. But then, we know that its decimal expansion is infinite. (In fact, it is really easy to see that any number with a finite decimal expansion is of the form n/10^m for some integers n and m...just multiply by 10 until the decimal point moves all the way to the end! So, the only numbers with finite decimal expansions are a special subset of rational numbers.) It is not clear to me whether the author is confused about this point or whether it is intended to demonstrate Sophie's mathematical innocence, but it just gets worse later. After a few entries on other subjects, she says:

I imagine that when she says "there is no exact value", she means "there is no exact representation of π as a ratio of whole numbers". (This might be how someone would indicate that in a diary entry for brevity, but it may not be sufficiently clear for an educational book.) Again, she repeats the strange idea that π might have only finitely many digits in its decimal expansion. The final part about the circumference being infinite is also a bit strange. I can see how this is poetic, and probably is supposed to sound like the imagination of a young girl. Of course, the circumference of a circle is finite, but since a circle has no end you can go around it infinitely many times. Poetic, but the suggestion that this is somehow connected to the infinite length of the decimal expansion of π is far from the sort of rigorous logical thinking that I imagine a mathematician as successful as Germain must have been good at.