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PositiveDefiniteMatrixQ

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

PositiveDefiniteMatrixQ[m]
    gives True if m is explicitly positive definite, and False
    otherwise.

A matrix m is positive definite if Re[Conjugate[x] . m . x] > 0
for every nonzero vector x.  Equivalently, the Hermitian part
(m + ConjugateTranspose[m]) / 2 has only positive eigenvalues.
The test is performed by attempting a Cholesky factorisation of
the Hermitian part; on numeric matrices this is dispatched to
BLAS/LAPACK's dpotrf (real) or zpotrf (complex) when available.
Returns False on non-numeric, non-square, ragged, empty, or
higher-rank tensor inputs.

Examples

All examples below are verified against the current Mathilda build.

In[1]:= PositiveDefiniteMatrixQ[{{5, -1}, {-1, 4}}]
Out[1]= True

In[2]:= PositiveDefiniteMatrixQ[{{2.3, -1.2}, {0.6, 3.7}}]
Out[2]= True

In[3]:= PositiveDefiniteMatrixQ[{{1, 2 I}, {-I, 4}}]
Out[3]= True

In[4]:= PositiveDefiniteMatrixQ[{{Pi, -5, 2}, {E, -3, -3}, {5, Sqrt[2], 5}}]
Out[4]= False

In[5]:= PositiveDefiniteMatrixQ[{{1, a}, {b, 2}}]
Out[5]= False

In[6]:= PositiveDefiniteMatrixQ[Table[1/(i + j - 1), {i, 8}, {j, 8}]]
Out[6]= True

Implementation notes

Algorithm. builtin_positive_definite_matrix_q tests whether Re[Conjugate[x].m.x] > 0 for all nonzero x, equivalently that the Hermitian part H = (m + ConjugateTranspose[m])/2 admits a Cholesky factorisation. After the square-matrix shape gate it coerces every entry to a (re, im) double (pdq_leaf_to_double: Integer, BigInt, Real, MPFR, exact Rational, Complex); any non-coercible leaf yields False (the predicate refuses claims it cannot prove). It builds H into the upper triangle of a column-major buffer (note: the input need not itself be Hermitian — only H matters), checks the diagonal is strictly positive, then runs Cholesky via LAPACK dpotrf/zpotrf (in-house pdq_chol_real_inplace/pdq_chol_complex_inplace fallback when USE_LAPACK is off). info == 0 ⇔ positive definite ⇒ True. Wrong arity emits PositiveDefiniteMatrixQ::argx.

  • Protected.
  • Equivalent to: the Hermitian part (m + ConjugateTranspose[m]) / 2

Attributes: Protected.

Implementation status

Stable — documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

References

Notes & additional examples

Worked examples

In[1]:= PositiveDefiniteMatrixQ[{{2, 1}, {1, 2}}]
Out[1]= True

In[1]:= PositiveDefiniteMatrixQ[{{1, 2}, {2, 1}}]
Out[1]= False

In[1]:= PositiveDefiniteMatrixQ[{{2, -1, 0}, {-1, 2, -1}, {0, -1, 2}}]
Out[1]= True

In[1]:= PositiveDefiniteMatrixQ[{{2, I}, {-I, 2}}]
Out[1]= True

Notes

PositiveDefiniteMatrixQ[m] tests whether the Hermitian part of m has only positive eigenvalues, equivalently whether Re[Conjugate[x] . m . x] > 0 for every nonzero vector x. It is decided by attempting a Cholesky factorisation of the Hermitian part. The second matrix {{1,2},{2,1}} has eigenvalues 3 and -1, so it is indefinite and the test returns False. The third example is the classic 3x3 second-difference (discrete Laplacian) matrix, which is positive definite. The last example is a genuinely Hermitian complex matrix {{2, I}, {-I, 2}} (note Conjugate[I] = -I off the diagonal) with eigenvalues 1 and 3; the complex path dispatches to LAPACK's zpotrf when available. The function returns False on non-numeric, non-square, ragged, or empty inputs.