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/* -- translated by f2c (version 20050501).
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You must link the resulting object file with libf2c:
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on Microsoft Windows system, link with libf2c.lib;
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on Linux or Unix systems, link with .../path/to/libf2c.a -lm
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or, if you install libf2c.a in a standard place, with -lf2c -lm
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-- in that order, at the end of the command line, as in
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Source for libf2c is in /netlib/f2c/libf2c.zip, e.g.,
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http://www.netlib.org/f2c/libf2c.zip
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#include "arpack_internal.h"
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/* Subroutine */ int igraphdlaev2_(doublereal *a, doublereal *b, doublereal *c__,
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doublereal *rt1, doublereal *rt2, doublereal *cs1, doublereal *sn1)
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/* System generated locals */
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/* Builtin functions */
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double sqrt(doublereal);
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static doublereal ab, df, cs, ct, tb, sm, tn, rt, adf, acs;
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static integer sgn1, sgn2;
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static doublereal acmn, acmx;
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/* -- LAPACK auxiliary routine (version 3.0) -- */
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/* Univ. of Tennessee, Univ. of California Berkeley, NAG Ltd., */
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/* Courant Institute, Argonne National Lab, and Rice University */
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/* October 31, 1992 */
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/* .. Scalar Arguments .. */
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/* DLAEV2 computes the eigendecomposition of a 2-by-2 symmetric matrix */
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/* On return, RT1 is the eigenvalue of larger absolute value, RT2 is the */
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/* eigenvalue of smaller absolute value, and (CS1,SN1) is the unit right */
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/* eigenvector for RT1, giving the decomposition */
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/* [ CS1 SN1 ] [ A B ] [ CS1 -SN1 ] = [ RT1 0 ] */
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/* [-SN1 CS1 ] [ B C ] [ SN1 CS1 ] [ 0 RT2 ]. */
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/* A (input) DOUBLE PRECISION */
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/* The (1,1) element of the 2-by-2 matrix. */
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/* B (input) DOUBLE PRECISION */
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/* The (1,2) element and the conjugate of the (2,1) element of */
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/* the 2-by-2 matrix. */
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/* C (input) DOUBLE PRECISION */
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/* The (2,2) element of the 2-by-2 matrix. */
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/* RT1 (output) DOUBLE PRECISION */
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/* The eigenvalue of larger absolute value. */
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/* RT2 (output) DOUBLE PRECISION */
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/* The eigenvalue of smaller absolute value. */
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/* CS1 (output) DOUBLE PRECISION */
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/* SN1 (output) DOUBLE PRECISION */
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/* The vector (CS1, SN1) is a unit right eigenvector for RT1. */
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/* RT1 is accurate to a few ulps barring over/underflow. */
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/* RT2 may be inaccurate if there is massive cancellation in the */
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/* determinant A*C-B*B; higher precision or correctly rounded or */
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/* correctly truncated arithmetic would be needed to compute RT2 */
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/* accurately in all cases. */
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/* CS1 and SN1 are accurate to a few ulps barring over/underflow. */
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/* Overflow is possible only if RT1 is within a factor of 5 of overflow. */
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/* Underflow is harmless if the input data is 0 or exceeds */
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/* underflow_threshold / macheps. */
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/* ===================================================================== */
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/* .. Parameters .. */
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/* .. Local Scalars .. */
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/* .. Intrinsic Functions .. */
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/* .. Executable Statements .. */
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/* Compute the eigenvalues */
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if (abs(*a) > abs(*c__)) {
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/* Computing 2nd power */
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rt = adf * sqrt(d__1 * d__1 + 1.);
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} else if (adf < ab) {
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/* Computing 2nd power */
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rt = ab * sqrt(d__1 * d__1 + 1.);
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/* Includes case AB=ADF=0 */
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*rt1 = (sm - rt) * .5;
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/* Order of execution important. */
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/* To get fully accurate smaller eigenvalue, */
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/* next line needs to be executed in higher precision. */
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*rt2 = acmx / *rt1 * acmn - *b / *rt1 * *b;
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} else if (sm > 0.) {
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*rt1 = (sm + rt) * .5;
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/* Order of execution important. */
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/* To get fully accurate smaller eigenvalue, */
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/* next line needs to be executed in higher precision. */
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*rt2 = acmx / *rt1 * acmn - *b / *rt1 * *b;
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/* Includes case RT1 = RT2 = 0 */
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/* Compute the eigenvector */
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*sn1 = 1. / sqrt(ct * ct + 1.);
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*cs1 = 1. / sqrt(tn * tn + 1.);
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} /* igraphdlaev2_ */
added
removed
src/lapack/dlaev2.c
