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Elements marked `*` need not be set and are not referenced by LAPACK routines.
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`AB` is a 4×5 matrix as `KL+KU+1 = 2+1+1 = 4`. Elements marked `*` need not be set and are not referenced by LAPACK routines.
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**Note:** When a band matrix is supplied for LU factorization, space must be allowed to store an additional `KL` superdiagonals, generated by fill-in as a result of row interchanges. This means that the matrix is stored according to the above scheme, but with `KL + KU` superdiagonals.
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**Note:** When a band matrix is supplied for LU factorization, space must be allowed to store an additional `KL` superdiagonals, which are generated by fill-in as a result of row interchanges. This means that the matrix is stored according to the above scheme, but with `KL + KU` superdiagonals.
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### Triangular Band Matrices
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Triangular band matrices are stored in the same format as general band matrices:
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- If `KL = 0`, the matrix is upper triangular
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- If `KU = 0`, the matrix is lower triangular
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- If `KL = 0`, the matrix is upper triangular.
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- If `KU = 0`, the matrix is lower triangular.
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### Symmetric or Hermitian Band Matrices
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For symmetric or Hermitian band matrices with `KD` subdiagonals or superdiagonals, only the upper or lower triangle (as specified by `UPLO`) needs to be stored.
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For symmetric or Hermitian band matrices with `KD` subdiagonals or superdiagonals, only the upper or lower triangle (as specified by an `UPLO` parameter) needs to be stored.
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**Upper Triangle Storage (UPLO = 'U'):**
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- Element `A( i, j )` is stored in `AB(KD+1+i-j, j)`
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- Element `A[i, j]` is stored in `AB[KD+1+i-j, j]`.
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- Valid range for `i`: `max(1, j-KD) <= i <= j`
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**Lower Triangle Storage (UPLO = 'L'):**
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- Element `A( i, j )` is stored in `AB(1+i-j, j)`
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- Valid range for `i`: `j <= i <= min(N, j+KD)`
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- Element `A[i, j]` is stored in `AB[1+i-j, j]`.
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- Valid range for `i`: `j <= i <= min(N, j+KD)`.
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#### Example (N=5, KD=2)
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#### Example
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For `UPLO = 'U'` (upper triangle stored):
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Let `N = 5` and `KD = 2`. Given the following matrix `A`,
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