SUBROUTINE DSVDCT( N, S, E, SHIFT, NUM ) * * -- LAPACK test routine (version 3.1) -- * Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. * November 2006 * * .. Scalar Arguments .. INTEGER N, NUM DOUBLE PRECISION SHIFT * .. * .. Array Arguments .. DOUBLE PRECISION E( * ), S( * ) * .. * * Purpose * ======= * * DSVDCT counts the number NUM of eigenvalues of a 2*N by 2*N * tridiagonal matrix T which are less than or equal to SHIFT. T is * formed by putting zeros on the diagonal and making the off-diagonals * equal to S(1), E(1), S(2), E(2), ... , E(N-1), S(N). If SHIFT is * positive, NUM is equal to N plus the number of singular values of a * bidiagonal matrix B less than or equal to SHIFT. Here B has diagonal * entries S(1), ..., S(N) and superdiagonal entries E(1), ... E(N-1). * If SHIFT is negative, NUM is equal to the number of singular values * of B greater than or equal to -SHIFT. * * See W. Kahan "Accurate Eigenvalues of a Symmetric Tridiagonal * Matrix", Report CS41, Computer Science Dept., Stanford University, * July 21, 1966 * * Arguments * ========= * * N (input) INTEGER * The dimension of the bidiagonal matrix B. * * S (input) DOUBLE PRECISION array, dimension (N) * The diagonal entries of the bidiagonal matrix B. * * E (input) DOUBLE PRECISION array of dimension (N-1) * The superdiagonal entries of the bidiagonal matrix B. * * SHIFT (input) DOUBLE PRECISION * The shift, used as described under Purpose. * * NUM (output) INTEGER * The number of eigenvalues of T less than or equal to SHIFT. * * ===================================================================== * * .. Parameters .. DOUBLE PRECISION ONE PARAMETER ( ONE = 1.0D0 ) DOUBLE PRECISION ZERO PARAMETER ( ZERO = 0.0D0 ) * .. * .. Local Scalars .. INTEGER I DOUBLE PRECISION M1, M2, MX, OVFL, SOV, SSHIFT, SSUN, SUN, TMP, \$ TOM, U, UNFL * .. * .. External Functions .. DOUBLE PRECISION DLAMCH EXTERNAL DLAMCH * .. * .. Intrinsic Functions .. INTRINSIC ABS, MAX, SQRT * .. * .. Executable Statements .. * * Get machine constants * UNFL = 2*DLAMCH( 'Safe minimum' ) OVFL = ONE / UNFL * * Find largest entry * MX = ABS( S( 1 ) ) DO 10 I = 1, N - 1 MX = MAX( MX, ABS( S( I+1 ) ), ABS( E( I ) ) ) 10 CONTINUE * IF( MX.EQ.ZERO ) THEN IF( SHIFT.LT.ZERO ) THEN NUM = 0 ELSE NUM = 2*N END IF RETURN END IF * * Compute scale factors as in Kahan's report * SUN = SQRT( UNFL ) SSUN = SQRT( SUN ) SOV = SQRT( OVFL ) TOM = SSUN*SOV IF( MX.LE.ONE ) THEN M1 = ONE / MX M2 = TOM ELSE M1 = ONE M2 = TOM / MX END IF * * Begin counting * U = ONE NUM = 0 SSHIFT = ( SHIFT*M1 )*M2 U = -SSHIFT IF( U.LE.SUN ) THEN IF( U.LE.ZERO ) THEN NUM = NUM + 1 IF( U.GT.-SUN ) \$ U = -SUN ELSE U = SUN END IF END IF TMP = ( S( 1 )*M1 )*M2 U = -TMP*( TMP / U ) - SSHIFT IF( U.LE.SUN ) THEN IF( U.LE.ZERO ) THEN NUM = NUM + 1 IF( U.GT.-SUN ) \$ U = -SUN ELSE U = SUN END IF END IF DO 20 I = 1, N - 1 TMP = ( E( I )*M1 )*M2 U = -TMP*( TMP / U ) - SSHIFT IF( U.LE.SUN ) THEN IF( U.LE.ZERO ) THEN NUM = NUM + 1 IF( U.GT.-SUN ) \$ U = -SUN ELSE U = SUN END IF END IF TMP = ( S( I+1 )*M1 )*M2 U = -TMP*( TMP / U ) - SSHIFT IF( U.LE.SUN ) THEN IF( U.LE.ZERO ) THEN NUM = NUM + 1 IF( U.GT.-SUN ) \$ U = -SUN ELSE U = SUN END IF END IF 20 CONTINUE RETURN * * End of DSVDCT * END