DNRM2 returns the euclidean norm of a vector via the function
 name, so that
    DNRM2 := sqrt( x'*x )

N

          N is INTEGER
         number of elements in input vector(s)

X

          X is DOUBLE PRECISION array, dimension ( 1 + ( N - 1 )*abs( INCX ) )

INCX

          INCX is INTEGER, storage spacing between elements of X
          If INCX > 0, X(1+(i-1)*INCX) = x(i) for 1 <= i <= n
          If INCX < 0, X(1-(n-i)*INCX) = x(i) for 1 <= i <= n
          If INCX = 0, x isn't a vector so there is no need to call
          this subroutine.  If you call it anyway, it will count x(1)
          in the vector norm N times.

Edward Anderson, Lockheed Martin

August 2016

Weslley Pereira, University of Colorado Denver, USA

  Anderson E. (2017)
  Algorithm 978: Safe Scaling in the Level 1 BLAS
  ACM Trans Math Softw 44:1--28
  https://doi.org/10.1145/3061665
  Blue, James L. (1978)
  A Portable Fortran Program to Find the Euclidean Norm of a Vector
  ACM Trans Math Softw 4:15--23
  https://doi.org/10.1145/355769.355771