ILAENV is called from the LAPACK routines to choose problem-dependent


 parameters for the local environment.  See ISPEC for a description of


 the parameters.


 ILAENV returns an INTEGER


 if ILAENV >= 0: ILAENV returns the value of the parameter specified by ISPEC


 if ILAENV < 0:  if ILAENV = -k, the k-th argument had an illegal value.


 This version provides a set of parameters which should give good,


 but not optimal, performance on many of the currently available


 computers.  Users are encouraged to modify this subroutine to set


 the tuning parameters for their particular machine using the option


 and problem size information in the arguments.


 This routine will not function correctly if it is converted to all


 lower case.  Converting it to all upper case is allowed.

ISPEC

          ISPEC is INTEGER


          Specifies the parameter to be returned as the value of


          ILAENV.


          = 1: the optimal blocksize; if this value is 1, an unblocked


               algorithm will give the best performance.


          = 2: the minimum block size for which the block routine


               should be used; if the usable block size is less than


               this value, an unblocked routine should be used.


          = 3: the crossover point (in a block routine, for N less


               than this value, an unblocked routine should be used)


          = 4: the number of shifts, used in the nonsymmetric


               eigenvalue routines (DEPRECATED)


          = 5: the minimum column dimension for blocking to be used;


               rectangular blocks must have dimension at least k by m,


               where k is given by ILAENV(2,...) and m by ILAENV(5,...)


          = 6: the crossover point for the SVD (when reducing an m by n


               matrix to bidiagonal form, if max(m,n)/min(m,n) exceeds


               this value, a QR factorization is used first to reduce


               the matrix to a triangular form.)


          = 7: the number of processors


          = 8: the crossover point for the multishift QR method


               for nonsymmetric eigenvalue problems (DEPRECATED)


          = 9: maximum size of the subproblems at the bottom of the


               computation tree in the divide-and-conquer algorithm


               (used by xGELSD and xGESDD)


          =10: ieee NaN arithmetic can be trusted not to trap


          =11: infinity arithmetic can be trusted not to trap


          12 <= ISPEC <= 16:


               xHSEQR or related subroutines,


               see IPARMQ for detailed explanation

NAME

          NAME is CHARACTER*(*)


          The name of the calling subroutine, in either upper case or


          lower case.

OPTS

          OPTS is CHARACTER*(*)


          The character options to the subroutine NAME, concatenated


          into a single character string.  For example, UPLO = 'U',


          TRANS = 'T', and DIAG = 'N' for a triangular routine would


          be specified as OPTS = 'UTN'.

N1

          N1 is INTEGER

N2

          N2 is INTEGER

N3

          N3 is INTEGER

N4

          N4 is INTEGER


          Problem dimensions for the subroutine NAME; these may not all


          be required.

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

November 2017

  The following conventions have been used when calling ILAENV from the


  LAPACK routines:


  1)  OPTS is a concatenation of all of the character options to


      subroutine NAME, in the same order that they appear in the


      argument list for NAME, even if they are not used in determining


      the value of the parameter specified by ISPEC.


  2)  The problem dimensions N1, N2, N3, N4 are specified in the order


      that they appear in the argument list for NAME.  N1 is used


      first, N2 second, and so on, and unused problem dimensions are


      passed a value of -1.


  3)  The parameter value returned by ILAENV is checked for validity in


      the calling subroutine.  For example, ILAENV is used to retrieve


      the optimal blocksize for STRTRI as follows:


      NB = ILAENV( 1, 'STRTRI', UPLO // DIAG, N, -1, -1, -1 )


      IF( NB.LE.1 ) NB = MAX( 1, N )