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// Copyright ©2023 The Gonum Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package gonum
// Dpttrs solves a tridiagonal system of the form
//
// A * X = B
//
// using the L*D*Lᵀ factorization of A computed by Dpttrf. D is a diagonal
// matrix specified in d, L is a unit bidiagonal matrix whose subdiagonal is
// specified in e, and X and B are n×nrhs matrices.
func (impl Implementation) Dpttrs(n, nrhs int, d, e []float64, b []float64, ldb int) {
switch {
case n < 0:
panic(nLT0)
case nrhs < 0:
panic(nrhsLT0)
case ldb < max(1, nrhs):
panic(badLdB)
}
// Quick return if possible.
if n == 0 || nrhs == 0 {
return
}
switch {
case len(d) < n:
panic(shortD)
case len(e) < n-1:
panic(shortE)
case len(b) < (n-1)*ldb+nrhs:
panic(shortB)
}
nb := 1
if nrhs > 1 {
nb = max(1, impl.Ilaenv(1, "DPTTRS", " ", n, nrhs, -1, -1))
}
if nb >= nrhs {
impl.dptts2(n, nrhs, d, e, b, ldb)
} else {
for j := 0; j < nrhs; j += nb {
jb := min(nrhs-j, nb)
impl.dptts2(n, jb, d, e, b[j:], ldb)
}
}
}
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