# 28 Numerics library [numerics]

## 28.9 Basic linear algebra algorithms [linalg]

### 28.9.7 Exposition-only helpers [linalg.helpers]

#### 28.9.7.6 Mandates [linalg.helpers.mandates]

[Note 1:
These exposition-only helper functions use the less constraining input concepts even for the output arguments, because the additional constraint for assignability of elements is not necessary, and they are sometimes used in a context where the third argument is an input type too.
— end note]
template<class MDS1, class MDS2> requires(is-mdspan<MDS1> && is-mdspan<MDS2>) constexpr bool compatible-static-extents(size_t r1, size_t r2) { // exposition only return MDS1::static_extent(r1) == dynamic_extent || MDS2::static_extent(r2) == dynamic_extent || MDS1::static_extent(r1) == MDS2::static_extent(r2)); } template<in-vector In1, in-vector In2, in-vector Out> constexpr bool possibly-addable() { // exposition only return compatible-static-extents<Out, In1>(0, 0) && compatible-static-extents<Out, In2>(0, 0) && compatible-static-extents<In1, In2>(0, 0); } template<in-matrix In1, in-matrix In2, in-matrix Out> constexpr bool possibly-addable() { // exposition only return compatible-static-extents<Out, In1>(0, 0) && compatible-static-extents<Out, In1>(1, 1) && compatible-static-extents<Out, In2>(0, 0) && compatible-static-extents<Out, In2>(1, 1) && compatible-static-extents<In1, In2>(0, 0) && compatible-static-extents<In1, In2>(1, 1); } template<in-matrix InMat, in-vector InVec, in-vector OutVec> constexpr bool possibly-multipliable() { // exposition only return compatible-static-extents<OutVec, InMat>(0, 0) && compatible-static-extents<InMat, InVec>(1, 0); } template<in-vector InVec, in-matrix InMat, in-vector OutVec> constexpr bool possibly-multipliable() { // exposition only return compatible-static-extents<OutVec, InMat>(0, 1) && compatible-static-extents<InMat, InVec>(0, 0); } template<in-matrix InMat1, in-matrix InMat2, in-matrix OutMat> constexpr bool possibly-multipliable() { // exposition only return compatible-static-extents<OutMat, InMat1>(0, 0) && compatible-static-extents<OutMat, InMat2>(1, 1) && compatible-static-extents<InMat1, InMat2>(1, 0); }