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author | yash1112 | 2017-07-07 21:20:49 +0530 |
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committer | yash1112 | 2017-07-07 21:20:49 +0530 |
commit | 3f52712f806fbd80d66dfdcaff401e5cf94dcca4 (patch) | |
tree | a8333b8187cb44b505b9fe37fc9a7ac8a1711c10 /src/fortran/lapack/dlaev2.f | |
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sci2c arduino updated
Diffstat (limited to 'src/fortran/lapack/dlaev2.f')
-rw-r--r-- | src/fortran/lapack/dlaev2.f | 169 |
1 files changed, 169 insertions, 0 deletions
diff --git a/src/fortran/lapack/dlaev2.f b/src/fortran/lapack/dlaev2.f new file mode 100644 index 0000000..49402fa --- /dev/null +++ b/src/fortran/lapack/dlaev2.f @@ -0,0 +1,169 @@ + SUBROUTINE DLAEV2( A, B, C, RT1, RT2, CS1, SN1 ) +* +* -- LAPACK auxiliary routine (version 3.1) -- +* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. +* November 2006 +* +* .. Scalar Arguments .. + DOUBLE PRECISION A, B, C, CS1, RT1, RT2, SN1 +* .. +* +* Purpose +* ======= +* +* DLAEV2 computes the eigendecomposition of a 2-by-2 symmetric matrix +* [ A B ] +* [ B C ]. +* On return, RT1 is the eigenvalue of larger absolute value, RT2 is the +* eigenvalue of smaller absolute value, and (CS1,SN1) is the unit right +* eigenvector for RT1, giving the decomposition +* +* [ CS1 SN1 ] [ A B ] [ CS1 -SN1 ] = [ RT1 0 ] +* [-SN1 CS1 ] [ B C ] [ SN1 CS1 ] [ 0 RT2 ]. +* +* Arguments +* ========= +* +* A (input) DOUBLE PRECISION +* The (1,1) element of the 2-by-2 matrix. +* +* B (input) DOUBLE PRECISION +* The (1,2) element and the conjugate of the (2,1) element of +* the 2-by-2 matrix. +* +* C (input) DOUBLE PRECISION +* The (2,2) element of the 2-by-2 matrix. +* +* RT1 (output) DOUBLE PRECISION +* The eigenvalue of larger absolute value. +* +* RT2 (output) DOUBLE PRECISION +* The eigenvalue of smaller absolute value. +* +* CS1 (output) DOUBLE PRECISION +* SN1 (output) DOUBLE PRECISION +* The vector (CS1, SN1) is a unit right eigenvector for RT1. +* +* Further Details +* =============== +* +* RT1 is accurate to a few ulps barring over/underflow. +* +* RT2 may be inaccurate if there is massive cancellation in the +* determinant A*C-B*B; higher precision or correctly rounded or +* correctly truncated arithmetic would be needed to compute RT2 +* accurately in all cases. +* +* CS1 and SN1 are accurate to a few ulps barring over/underflow. +* +* Overflow is possible only if RT1 is within a factor of 5 of overflow. +* Underflow is harmless if the input data is 0 or exceeds +* underflow_threshold / macheps. +* +* ===================================================================== +* +* .. Parameters .. + DOUBLE PRECISION ONE + PARAMETER ( ONE = 1.0D0 ) + DOUBLE PRECISION TWO + PARAMETER ( TWO = 2.0D0 ) + DOUBLE PRECISION ZERO + PARAMETER ( ZERO = 0.0D0 ) + DOUBLE PRECISION HALF + PARAMETER ( HALF = 0.5D0 ) +* .. +* .. Local Scalars .. + INTEGER SGN1, SGN2 + DOUBLE PRECISION AB, ACMN, ACMX, ACS, ADF, CS, CT, DF, RT, SM, + $ TB, TN +* .. +* .. Intrinsic Functions .. + INTRINSIC ABS, SQRT +* .. +* .. Executable Statements .. +* +* Compute the eigenvalues +* + SM = A + C + DF = A - C + ADF = ABS( DF ) + TB = B + B + AB = ABS( TB ) + IF( ABS( A ).GT.ABS( C ) ) THEN + ACMX = A + ACMN = C + ELSE + ACMX = C + ACMN = A + END IF + IF( ADF.GT.AB ) THEN + RT = ADF*SQRT( ONE+( AB / ADF )**2 ) + ELSE IF( ADF.LT.AB ) THEN + RT = AB*SQRT( ONE+( ADF / AB )**2 ) + ELSE +* +* Includes case AB=ADF=0 +* + RT = AB*SQRT( TWO ) + END IF + IF( SM.LT.ZERO ) THEN + RT1 = HALF*( SM-RT ) + SGN1 = -1 +* +* Order of execution important. +* To get fully accurate smaller eigenvalue, +* next line needs to be executed in higher precision. +* + RT2 = ( ACMX / RT1 )*ACMN - ( B / RT1 )*B + ELSE IF( SM.GT.ZERO ) THEN + RT1 = HALF*( SM+RT ) + SGN1 = 1 +* +* Order of execution important. +* To get fully accurate smaller eigenvalue, +* next line needs to be executed in higher precision. +* + RT2 = ( ACMX / RT1 )*ACMN - ( B / RT1 )*B + ELSE +* +* Includes case RT1 = RT2 = 0 +* + RT1 = HALF*RT + RT2 = -HALF*RT + SGN1 = 1 + END IF +* +* Compute the eigenvector +* + IF( DF.GE.ZERO ) THEN + CS = DF + RT + SGN2 = 1 + ELSE + CS = DF - RT + SGN2 = -1 + END IF + ACS = ABS( CS ) + IF( ACS.GT.AB ) THEN + CT = -TB / CS + SN1 = ONE / SQRT( ONE+CT*CT ) + CS1 = CT*SN1 + ELSE + IF( AB.EQ.ZERO ) THEN + CS1 = ONE + SN1 = ZERO + ELSE + TN = -CS / TB + CS1 = ONE / SQRT( ONE+TN*TN ) + SN1 = TN*CS1 + END IF + END IF + IF( SGN1.EQ.SGN2 ) THEN + TN = CS1 + CS1 = -SN1 + SN1 = TN + END IF + RETURN +* +* End of DLAEV2 +* + END |