diff --git a/lapack/testlapack/dtrexc.go b/lapack/testlapack/dtrexc.go
index 59d0d3d0..c9eda4ac 100644
--- a/lapack/testlapack/dtrexc.go
+++ b/lapack/testlapack/dtrexc.go
@@ -6,6 +6,7 @@ package testlapack
 
 import (
 	"fmt"
+	"math"
 	"math/cmplx"
 	"testing"
 
@@ -23,60 +24,68 @@ type Dtrexcer interface {
 func DtrexcTest(t *testing.T, impl Dtrexcer) {
 	rnd := rand.New(rand.NewSource(1))
 
-	for _, compq := range []lapack.UpdateSchurComp{lapack.UpdateSchurNone, lapack.UpdateSchur} {
-		for _, n := range []int{1, 2, 3, 4, 5, 6, 10, 18, 31, 53} {
-			for _, extra := range []int{0, 1, 11} {
-				for cas := 0; cas < 100; cas++ {
-					tmat, _, _ := randomSchurCanonical(n, n+extra, rnd)
-					ifst := rnd.Intn(n)
-					ilst := rnd.Intn(n)
-					testDtrexc(t, impl, compq, tmat, ifst, ilst, extra, rnd)
+	for _, n := range []int{0, 1, 2, 3, 4, 5, 6, 10, 18, 31, 53} {
+		for _, extra := range []int{0, 3} {
+			for cas := 0; cas < 100; cas++ {
+				var ifst, ilst int
+				if n > 0 {
+					ifst = rnd.Intn(n)
+					ilst = rnd.Intn(n)
 				}
+				dtrexcTest(t, impl, rnd, n, ifst, ilst, extra)
 			}
 		}
 	}
-
-	for _, compq := range []lapack.UpdateSchurComp{lapack.UpdateSchurNone, lapack.UpdateSchur} {
-		for _, extra := range []int{0, 1, 11} {
-			tmat, _, _ := randomSchurCanonical(0, extra, rnd)
-			testDtrexc(t, impl, compq, tmat, 0, 0, extra, rnd)
-		}
-	}
 }
 
-func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat blas64.General, ifst, ilst, extra int, rnd *rand.Rand) {
+func dtrexcTest(t *testing.T, impl Dtrexcer, rnd *rand.Rand, n, ifst, ilst, extra int) {
 	const tol = 1e-13
 
-	n := tmat.Rows
-	fstSize, fstFirst := schurBlockSize(tmat, ifst)
-	lstSize, lstFirst := schurBlockSize(tmat, ilst)
+	tmatCopy, _, _ := randomSchurCanonical(n, n+extra, false, rnd)
+	tmat1 := cloneGeneral(tmatCopy)
+	tmat2 := cloneGeneral(tmatCopy)
 
-	tmatCopy := cloneGeneral(tmat)
+	fstSize, fstFirst := schurBlockSize(tmat1, ifst)
+	lstSize, lstFirst := schurBlockSize(tmat1, ilst)
 
-	var wantq bool
-	var q, qCopy blas64.General
-	if compq == lapack.UpdateSchur {
-		wantq = true
-		q = eye(n, n+extra)
-		qCopy = cloneGeneral(q)
-	}
+	name := fmt.Sprintf("Case n=%v,ifst=%v,nbfst=%v,ilst=%v,nblst=%v,extra=%v",
+		n, ifst, fstSize, ilst, lstSize, extra)
 
+	// 1. Test without accumulating Q.
+
+	compq := lapack.UpdateSchurNone
 	work := nanSlice(n)
+	ifstGot1, ilstGot1, ok1 := impl.Dtrexc(compq, n, tmat1.Data, tmat1.Stride, nil, 1, ifst, ilst, work)
 
-	ifstGot, ilstGot, ok := impl.Dtrexc(compq, n, tmat.Data, tmat.Stride, q.Data, max(1, q.Stride), ifst, ilst, work)
-
-	prefix := fmt.Sprintf("Case compq=%v, n=%v, ifst=%v, nbf=%v, ilst=%v, nbl=%v, extra=%v",
-		compq, n, ifst, fstSize, ilst, lstSize, extra)
-
-	if !generalOutsideAllNaN(tmat) {
-		t.Errorf("%v: out-of-range write to T", prefix)
-	}
-	if wantq && !generalOutsideAllNaN(q) {
-		t.Errorf("%v: out-of-range write to Q", prefix)
+	if !generalOutsideAllNaN(tmat1) {
+		t.Errorf("%v: out-of-range write to T1", name)
 	}
 
-	if !ok {
-		t.Logf("%v: Dtrexc returned ok=false", prefix)
+	// 2. Test with accumulating Q.
+
+	q := eye(n, n+extra)
+	qCopy := cloneGeneral(q)
+
+	compq = lapack.UpdateSchur
+	work = nanSlice(n)
+	ifstGot2, ilstGot2, ok2 := impl.Dtrexc(compq, n, tmat2.Data, tmat2.Stride, q.Data, q.Stride, ifst, ilst, work)
+
+	if !generalOutsideAllNaN(q) {
+		t.Errorf("%v: out-of-range write to Q", name)
+	}
+
+	// Check that outputs from cases 1. and 2. are exactly equal, then check one of them.
+	if ifstGot1 != ifstGot2 {
+		t.Errorf("%v: ifstGot1 != ifstGot2", name)
+	}
+	if ilstGot1 != ilstGot2 {
+		t.Errorf("%v: ilstGot1 != ilstGot2", name)
+	}
+	if ok1 != ok2 {
+		t.Errorf("%v: ok1 != ok2", name)
+	}
+	if !equalGeneral(tmat1, tmat2) {
+		t.Errorf("%v: T1 != T2", name)
 	}
 
 	// Check that the index of the first block was correctly updated (if
@@ -85,8 +94,8 @@ func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat
 	if !fstFirst {
 		ifstWant = ifst - 1
 	}
-	if ifstWant != ifstGot {
-		t.Errorf("%v: unexpected ifst index. Want %v, got %v ", prefix, ifstWant, ifstGot)
+	if ifstWant != ifstGot1 {
+		t.Errorf("%v: unexpected ifst=%v, want %v", name, ifstGot1, ifstWant)
 	}
 
 	// Check that the index of the last block is as expected when ok=true.
@@ -96,7 +105,7 @@ func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat
 	if !lstFirst {
 		ilstWant--
 	}
-	if ok {
+	if ok1 {
 		if ifstWant < ilstWant {
 			// If the blocks are swapped backwards, these
 			// adjustments are not necessary, the first row of the
@@ -108,43 +117,33 @@ func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat
 				ilstWant++
 			}
 		}
-		if ilstWant != ilstGot {
-			t.Errorf("%v: unexpected ilst index. Want %v, got %v", prefix, ilstWant, ilstGot)
+		if ilstWant != ilstGot1 {
+			t.Errorf("%v: unexpected ilst=%v, want %v", name, ilstGot1, ilstWant)
 		}
 	}
 
-	if n <= 1 || ifstGot == ilstGot {
+	if n <= 1 || ifstGot1 == ilstGot1 {
 		// Too small matrix or no swapping.
 		// Check that T was not modified.
-		for i := 0; i < n; i++ {
-			for j := 0; j < n; j++ {
-				if tmat.Data[i*tmat.Stride+j] != tmatCopy.Data[i*tmatCopy.Stride+j] {
-					t.Errorf("%v: unexpected modification at T[%v,%v]", prefix, i, j)
-				}
-			}
-		}
-		if !wantq {
-			return
+		if !equalGeneral(tmat1, tmatCopy) {
+			t.Errorf("%v: unexpected modification of T when no swapping", name)
 		}
 		// Check that Q was not modified.
-		for i := 0; i < n; i++ {
-			for j := 0; j < n; j++ {
-				if q.Data[i*q.Stride+j] != qCopy.Data[i*qCopy.Stride+j] {
-					t.Errorf("%v: unexpected modification at Q[%v,%v]", prefix, i, j)
-				}
-			}
+		if !equalGeneral(q, qCopy) {
+			t.Errorf("%v: unexpected modification of Q when no swapping", name)
 		}
+		// Nothing more to check
 		return
 	}
 
-	if !isSchurCanonicalGeneral(tmat) {
-		t.Errorf("%v: T is not in Schur canonical form", prefix)
+	if !isSchurCanonicalGeneral(tmat1) {
+		t.Errorf("%v: T is not in Schur canonical form", name)
 	}
 
 	// Check that T was not modified except above the second subdiagonal in
 	// rows and columns [modMin,modMax].
-	modMin := min(ifstGot, ilstGot)
-	modMax := max(ifstGot, ilstGot) + fstSize
+	modMin := min(ifstGot1, ilstGot1)
+	modMax := max(ifstGot1, ilstGot1) + fstSize
 	for i := 0; i < n; i++ {
 		for j := 0; j < n; j++ {
 			if modMin <= i && i < modMax && j+1 >= i {
@@ -153,9 +152,9 @@ func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat
 			if modMin <= j && j < modMax && j+1 >= i {
 				continue
 			}
-			diff := tmat.Data[i*tmat.Stride+j] - tmatCopy.Data[i*tmatCopy.Stride+j]
+			diff := tmat1.Data[i*tmat1.Stride+j] - tmatCopy.Data[i*tmatCopy.Stride+j]
 			if diff != 0 {
-				t.Errorf("%v: unexpected modification at T[%v,%v]", prefix, i, j)
+				t.Errorf("%v: unexpected modification at T[%v,%v]", name, i, j)
 			}
 		}
 	}
@@ -163,40 +162,35 @@ func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat
 	// Check that the block at ifstGot was delivered to ilstGot correctly.
 	if fstSize == 1 {
 		// 1×1 blocks are swapped exactly.
-		got := tmat.Data[ilstGot*tmat.Stride+ilstGot]
-		want := tmatCopy.Data[ifstGot*tmatCopy.Stride+ifstGot]
+		got := tmat1.Data[ilstGot1*tmat1.Stride+ilstGot1]
+		want := tmatCopy.Data[ifstGot1*tmatCopy.Stride+ifstGot1]
 		if want != got {
 			t.Errorf("%v: unexpected 1×1 block at T[%v,%v]. Want %v, got %v",
-				prefix, want, got, ilstGot, ilstGot)
+				name, want, got, ilstGot1, ilstGot1)
 		}
 	} else {
-		// Check that the swapped 2×2 block is in Schur canonical form.
-		a, b, c, d := extract2x2Block(tmat.Data[ilstGot*tmat.Stride+ilstGot:], tmat.Stride)
-		if !isSchurCanonical(a, b, c, d) {
-			t.Errorf("%v: 2×2 block at T[%v,%v] not in Schur canonical form", prefix, ilstGot, ilstGot)
-		}
-		ev1Got, ev2Got := schurBlockEigenvalues(a, b, c, d)
-
 		// Check that the swapped 2×2 block has the same eigenvalues.
+		a, b, c, d := extract2x2Block(tmat1.Data[ilstGot1*tmat1.Stride+ilstGot1:], tmat1.Stride)
+		ev1Got, ev2Got := schurBlockEigenvalues(a, b, c, d)
 		// The block was originally located at T[ifstGot,ifstGot].
-		a, b, c, d = extract2x2Block(tmatCopy.Data[ifstGot*tmatCopy.Stride+ifstGot:], tmatCopy.Stride)
+		a, b, c, d = extract2x2Block(tmatCopy.Data[ifstGot1*tmatCopy.Stride+ifstGot1:], tmatCopy.Stride)
 		ev1Want, ev2Want := schurBlockEigenvalues(a, b, c, d)
-		if cmplx.Abs(ev1Got-ev1Want) > tol {
-			t.Errorf("%v: unexpected first eigenvalue of 2×2 block at T[%v,%v]. Want %v, got %v",
-				prefix, ilstGot, ilstGot, ev1Want, ev1Got)
+		diff := cmplx.Abs(ev1Got - ev1Want)
+		if diff > tol {
+			t.Errorf("%v: unexpected first eigenvalue of 2×2 block [%v %v; %v %v] at T[%v,%v]; diff=%v, want<=%v",
+				name, a, b, c, d, ilstGot1, ilstGot1, diff, tol)
 		}
-		if cmplx.Abs(ev2Got-ev2Want) > tol {
-			t.Errorf("%v: unexpected second eigenvalue of 2×2 block at T[%v,%v]. Want %v, got %v",
-				prefix, ilstGot, ilstGot, ev2Want, ev2Got)
+		diff = cmplx.Abs(ev2Got - ev2Want)
+		if diff > tol {
+			t.Errorf("%v: unexpected second eigenvalue of 2×2 block [%v %v; %v %v] at T[%v,%v]. Want %v, got %v",
+				name, a, b, c, d, ilstGot1, ilstGot1, diff, tol)
 		}
 	}
 
-	if !wantq {
-		return
-	}
-
-	if !isOrthogonal(q) {
-		t.Errorf("%v: Q is not orthogonal", prefix)
+	// Check that Q is orthogonal.
+	resid := residualOrthogonal(q, false)
+	if resid > float64(n)*tol {
+		t.Errorf("%v: Q is not orthogonal; resid=%v, want<=%v", name, resid, float64(n)*tol)
 	}
 	// Check that Q is unchanged outside of columns [modMin,modMax].
 	for i := 0; i < n; i++ {
@@ -205,16 +199,126 @@ func testDtrexc(t *testing.T, impl Dtrexcer, compq lapack.UpdateSchurComp, tmat
 				continue
 			}
 			if q.Data[i*q.Stride+j]-qCopy.Data[i*qCopy.Stride+j] != 0 {
-				t.Errorf("%v: unexpected modification of Q[%v,%v]", prefix, i, j)
+				t.Errorf("%v: unexpected modification of Q[%v,%v]", name, i, j)
 			}
 		}
 	}
-	// Check that Qᵀ TOrig Q == T.
-	tq := eye(n, n)
-	blas64.Gemm(blas.NoTrans, blas.NoTrans, 1, tmatCopy, q, 0, tq)
-	qtq := eye(n, n)
-	blas64.Gemm(blas.Trans, blas.NoTrans, 1, q, tq, 0, qtq)
-	if !equalApproxGeneral(qtq, tmat, tol) {
-		t.Errorf("%v: Qᵀ (initial T) Q and (final T) are not equal", prefix)
+	// Check that Qᵀ * TOrig * Q == T
+	qt := zeros(n, n, n)
+	blas64.Gemm(blas.Trans, blas.NoTrans, 1, q, tmatCopy, 0, qt)
+	qtq := cloneGeneral(tmat1)
+	blas64.Gemm(blas.NoTrans, blas.NoTrans, -1, qt, q, 1, qtq)
+	resid = dlange(lapack.MaxColumnSum, n, n, qtq.Data, qtq.Stride)
+	if resid > float64(n)*tol {
+		t.Errorf("%v: mismatch between Qᵀ*(initial T)*Q and (final T); resid=%v, want<=%v",
+			name, resid, float64(n)*tol)
+	}
+}
+
+func residualOrthogonal(q blas64.General, rowwise bool) float64 {
+	m, n := q.Rows, q.Cols
+	if m == 0 || n == 0 {
+		return 0
+	}
+	var transq blas.Transpose
+	if m < n || (m == n && rowwise) {
+		transq = blas.NoTrans
+	} else {
+		transq = blas.Trans
+	}
+	minmn := min(m, n)
+
+	// Set work = I.
+	work := blas64.Symmetric{
+		Uplo:   blas.Upper,
+		N:      minmn,
+		Data:   make([]float64, minmn*minmn),
+		Stride: minmn,
+	}
+	for i := 0; i < minmn; i++ {
+		work.Data[i*work.Stride+i] = 1
+	}
+
+	// Compute
+	//  work = work - Q * Qᵀ = I - Q * Qᵀ
+	// or
+	//  work = work - Qᵀ * Q = I - Qᵀ * Q
+	blas64.Syrk(transq, -1, q, 1, work)
+	return dlansy(lapack.MaxColumnSum, blas.Upper, work.N, work.Data, work.Stride)
+}
+
+func dlansy(norm lapack.MatrixNorm, uplo blas.Uplo, n int, a []float64, lda int) float64 {
+	if n == 0 {
+		return 0
+	}
+	work := make([]float64, n)
+	switch norm {
+	case lapack.MaxAbs:
+		if uplo == blas.Upper {
+			var max float64
+			for i := 0; i < n; i++ {
+				for j := i; j < n; j++ {
+					v := math.Abs(a[i*lda+j])
+					if math.IsNaN(v) {
+						return math.NaN()
+					}
+					if v > max {
+						max = v
+					}
+				}
+			}
+			return max
+		}
+		var max float64
+		for i := 0; i < n; i++ {
+			for j := 0; j <= i; j++ {
+				v := math.Abs(a[i*lda+j])
+				if math.IsNaN(v) {
+					return math.NaN()
+				}
+				if v > max {
+					max = v
+				}
+			}
+		}
+		return max
+	case lapack.MaxRowSum, lapack.MaxColumnSum:
+		// A symmetric matrix has the same 1-norm and ∞-norm.
+		for i := 0; i < n; i++ {
+			work[i] = 0
+		}
+		if uplo == blas.Upper {
+			for i := 0; i < n; i++ {
+				work[i] += math.Abs(a[i*lda+i])
+				for j := i + 1; j < n; j++ {
+					v := math.Abs(a[i*lda+j])
+					work[i] += v
+					work[j] += v
+				}
+			}
+		} else {
+			for i := 0; i < n; i++ {
+				for j := 0; j < i; j++ {
+					v := math.Abs(a[i*lda+j])
+					work[i] += v
+					work[j] += v
+				}
+				work[i] += math.Abs(a[i*lda+i])
+			}
+		}
+		var max float64
+		for i := 0; i < n; i++ {
+			v := work[i]
+			if math.IsNaN(v) {
+				return math.NaN()
+			}
+			if v > max {
+				max = v
+			}
+		}
+		return max
+	default:
+		// lapack.Frobenius:
+		panic("not implemented")
 	}
 }