[linalg.layout.packed.obs]

constexpr index_type required_span_size() const noexcept;

Returns: extents_.extent(0) * (extents_.extent(0) + 1)/2.

[Note 1:

For example, a 5 x 5 packed matrix only stores 15 matrix elements.

— end note]

template<class Index0, class Index1>
  constexpr index_type operator() (Index0 ind0, Index1 ind1) const noexcept;

Let i be extents_type::index-cast(ind0), and let j be extents_type::index-cast(ind1).

Preconditions: i, j is a multidimensional index in extents_ ([mdspan.overview]).

Returns: Let N be extents_.extent(0).

Then

(5.1)
(*this)(j, i) if i > j is true; otherwise
(5.2)
i + j * (j + 1)/2 if is_same_v<StorageOrder, column_major_t> && is_same_v<Triangle, upper_triangle_t> is true or is_same_v<StorageOrder, row_major_t> && is_same_v<Triangle, lower_triangle_t> is true; otherwise
(5.3)
j + N * i - i * (i + 1)/2.

constexpr index_type stride(rank_type r) const noexcept;

Returns: 1.

template<class OtherExtents>
  friend constexpr bool operator=(const mapping& x, const mapping<OtherExtents>& y) noexcept;

Effects: Equivalent to: return x.extents() == y.extents();

29 Numerics library [numerics]