I am currently doing a project in which I intend to include the most insightful possible proof of the Hasse–Minkowski theorem (also known as the Hasse principle for quadratic forms, among other names) over $\mathbb{Q}$, as well as a separate proof of the theorem over all global fields.

In order to do this, I first want to compare all the proofs given in the existing literature. So far, I have managed to compile the following list of books and papers, in which proofs of the theorem can be found:

• Z.I. Borevich and I.R. Shafarevich: Number Theory (1964)
• J.W.S. Cassels: Rational Quadratic Forms (1978)
• J.W.S. Cassels: Lectures on Elliptic Curves (1991) [in which the theorem is only proved in the case of three variables]
• H. Hasse: Über die Darstellbarkeit von Zahlen durch quadratische Formen im Körper der rationalen Zahlen (1923); Über die Äquivalenz quadraticher Formen im Körper der rationalen Zahlen (1923); Symmetrische Matrizen im Körper der rationalen Zahlen (1924); Darstellbarkeit von Zahlen durch quadratische Formen in einem beliebigen algebraischen Zahlkörper (1924) [the proof here extending through four papers and applying to all algebraic number fields]
• Y. Kitaoka: Arithmetic of Quadratic Forms (1993)
• T.Y. Lam: The Algebraic Theory of Quadratic Forms (1973)
• O.T. O'Meara: Introduction to Quadratic Forms (1963) [in which the theorem is proved for all global fields]
• J.-P. Serre: Cours d'arithmétique (1970)
• G. Shimura: Arithmetic of Quadratic Forms (2010)

The proofs given in the above resources are all quite remarkably dissimilar in certain cases, which is encouraging, in as far as it suggests that the "canonical" proof of the theorem has yet to be established.

I would hence like to ask the members of the MathOverflow community if they are aware of any other proofs of the theorem, and if they could direct me to where they can be found.

• I am looking especially for proofs of the theorem in the more general case over all algebraic number fields, as, apart from Hasse's original, the only full proofs of this that I have been able to find are those in the grand treatises of Lam and O'Meara.
• Proofs of the "weak" Hasse–Minkowski theorem (i.e. that pertaining to the equivalence of quadratic forms as opposed to their representing $0$) in the case of fields where the "strong" Hasse–Minkowski theorem does not hold are also especially welcome.
• Proofs of the theorem for a particular $n \geq 3$ are also very welcome ($n$ here denoting the number of variables of the quadratic forms).
• The proofs do not have to be ones found in published books: Proofs from e.g. lecture notes are also very welcome, provided they are not entirely based on a proof given in a published book.

Edit: The answers to this post have helped identify the following resources, in which the theorem is either proved in its entirety or otherwise meaningfully discussed in some manner (works labelled with an asterisk are ones of which I have thus far been unable to obtain a copy):

• J.W.S. Cassels: Note on Quadratic Forms Over the Rational Field (1959)
• L.E. Dickson: Studies in the Theory of Numbers (1930)*
• M. Eichler: Quadratische Formen und orthogonale Gruppen (1952)*
• A. Gamzon: The Hasse–Minkowski Theorem (2006) [an honours thesis written at the University of Connecticut under the supervision of Keith Conrad]
• R. Heath-Brown: A New Form of the Circle Method, and its Application to Quadratic Forms (1996)
• B.W. Jones: The Arithmetical Theory of Quadratic Forms (1950)
• C.L. Siegel: Equivalence of Quadratic Forms (1948)
• Th. Skolem: Diophantische Gleichungen (1938)*
• T.A. Springer: Note on Quadratic Forms over Algebraic Number Fields (1957)*
• G.L. Watson: Integral Quadratic Forms (1960)*
• E. Witt: Theorie der quadratischen Formen in beliebigen Körpern (1937)

I have decided to keep the works identified after the creation of this post on this separate list. I am currently in the process of reviewing all of the above resources, and will continually be amending the list with new entries and commentary on existing entries.

Edit 30 July 2021: I have identified the following classical proofs of the theorem over $\mathbb{Q}$ in the case of $n=3$ (where the theorem is given in Legendre’s classical pre-$p$-adic formulation in terms of congruence conditions for the coefficients):

• P.G.L. Dirichlet: Vorlesungen Über Zahlentheorie, 2. Aufl. (1871) [§§156-157]
• C.F. Gauss: Disquisitiones Arithmeticae (1801) [§§293-298]
• A.-M. Legendre: Essai sur la Théorie des Nombres (1798) [§IV]

For the proof of the theorem over all algebraic number fields, the $n=2$ case of course follows from the global square theorem over algebraic number fields, which was first proved in:

• D. Hilbert: Über die Theorie des relativquadratischen Körpers (1898)

The $n=3$ case was originally proved by Furtwängler (albeit in a different formulation) in:

• Ph. Furtwängler: Über die Reziprozitätsgesetze für ungerade Primzahlexponenten, Parts I-III (1909, 1912 and 1913 resp.)

These works were cited by Hasse in his:

• H. Hasse: Darstellbarkeit von Zahlen durch quadratische Formen in einem beliebigen algebraischen Zahlkörper (1924) [also listed above]

For $n=4$, there is of course Hasse’s original proof, the books of Lam and O’Meara and the following paper by Springer:

• T.A. Springer: Note on Quadratic Forms over Algebraic Number Fields (1957) [also listed above]

I have still not managed to obtain a copy of this paper. – It appears to have been published in a journal called Indagationes Mathematicae. If anyone happens to know if this journal or this paper specifically has been digitalised somewhere, all information would be greatly appreciated.