The pseudo-determinant of a square n-by-n matrix A may be defined as:

${\displaystyle |\mathbf {A} |_{+}=\lim _{\alpha \to 0}{\frac {|\mathbf {A} +\alpha \mathbf {I} |}{\alpha ^{n-\operatorname {rank} (\mathbf {A} )}}}}$ 

where |A| denotes the usual determinant, I denotes the identity matrix and rank(A) denotes the rank of A.^[2]

The Vahlen matrix of a conformal transformation, the Möbius transformation (i.e. ${\displaystyle (ax+b)(cx+d)^{-1}}$  for ${\displaystyle a,b,c,d\in {\mathcal {G}}(p,q)}$ ), is defined as ${\displaystyle [f]={\begin{bmatrix}a&b\\c&d\end{bmatrix}}}$ . By the pseudo-determinant of the Vahlen matrix for the conformal transformation, we mean

${\displaystyle \operatorname {pdet} {\begin{bmatrix}a&b\\c&d\end{bmatrix}}=ad^{\dagger }-bc^{\dagger }.}$ 

If ${\displaystyle \operatorname {pdet} [f]>0}$ , the transformation is sense-preserving (rotation) whereas if the ${\displaystyle \operatorname {pdet} [f]<0}$ , the transformation is sense-preserving (reflection).

If ${\displaystyle A}$  is positive semi-definite, then the singular values and eigenvalues of ${\displaystyle A}$  coincide. In this case, if the singular value decomposition (SVD) is available, then ${\displaystyle |\mathbf {A} |_{+}}$  may be computed as the product of the non-zero singular values. If all singular values are zero, then the pseudo-determinant is 1.

Supposing ${\displaystyle \operatorname {rank} (A)=k}$ , so that k is the number of non-zero singular values, we may write ${\displaystyle A=PP^{\dagger }}$  where ${\displaystyle P}$  is some n-by-k matrix and the dagger is the conjugate transpose. The singular values of ${\displaystyle A}$  are the squares of the singular values of ${\displaystyle P}$  and thus we have ${\displaystyle |A|_{+}=\left|P^{\dagger }P\right|}$ , where ${\displaystyle \left|P^{\dagger }P\right|}$  is the usual determinant in k dimensions. Further, if ${\displaystyle P}$  is written as the block column ${\displaystyle P=\left({\begin{smallmatrix}C\\D\end{smallmatrix}}\right)}$ , then it holds, for any heights of the blocks ${\displaystyle C}$  and ${\displaystyle D}$ , that ${\displaystyle |A|_{+}=\left|C^{\dagger }C+D^{\dagger }D\right|}$ .

If a statistical procedure ordinarily compares distributions in terms of the determinants of variance-covariance matrices then, in the case of singular matrices, this comparison can be undertaken by using a combination of the ranks of the matrices and their pseudo-determinants, with the matrix of higher rank being counted as "largest" and the pseudo-determinants only being used if the ranks are equal.^[3] Thus pseudo-determinants are sometime presented in the outputs of statistical programs in cases where covariance matrices are singular.^[4]

• Matrix determinant
• Moore–Penrose pseudoinverse, which can also be obtained in terms of the non-zero singular values.