?larra

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 4/28/2026

Version

Public

Computes the splitting points with the specified threshold.

call slarra ( n , d , e , e2 , spltol , tnrm , nsplit , isplit , info )

call dlarra ( n , d , e , e2 , spltol , tnrm , nsplit , isplit , info )

slarra dlarra

The routine computes the splitting points with the specified threshold and sets any “small” off-diagonal elements to zero.

n: INTEGER . The order of the matrix ( n> 1 ).
d: REAL for slarra DOUBLE PRECISION for dlarra Array, DIMENSION ( n ). Contains n diagonal elements of the tridiagonal matrix T .
e: REAL for slarra DOUBLE PRECISION for dlarra Array, DIMENSION ( n ). First (n-1) entries contain the subdiagonal elements of the tridiagonal matrix T ; e(n) need not be set.
e2: REAL for slarra DOUBLE PRECISION for dlarra Array, DIMENSION ( n ). First (n-1) entries contain the squares of the subdiagonal elements of the tridiagonal matrix T ; e2(n) need not be set.
spltol: REAL for slarra DOUBLE PRECISION for dlarra The threshold for splitting. Two criteria can be used: spltol<0 : criterion based on absolute off-diagonal value; spltol >0 : criterion that preserves relative accuracy.
tnrm: REAL for slarra DOUBLE PRECISION for dlarra The norm of the matrix.

e: On exit, the entries e(isplit(i)), 1 ≤ i ≤ nsplit , are set to zero, the other entries of e are untouched.
e2: On exit, the entries e2(isplit(i)), 1 ≤ i ≤ nsplit , are set to zero.
nsplit: INTEGER . The number of blocks the matrix T splits into. 1 ≤ nsplit ≤ n
isplit: Array, DIMENSION ( n ). The splitting points, at which T breaks up into blocks. The first block consists of rows/columns 1 to isplit(1) , the second of rows/columns isplit(1)+1 through isplit(2) , and so on, and the nsplit -th consists of rows/columns isplit(nsplit-1)+1 through isplit(nsplit)=n .

info

INTEGER .