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# skbio.stats.composition.alr_inv

`skbio.stats.composition.``alr_inv`(mat, denominator_idx=0)

  State: Experimental as of 0.5.5. Performs inverse additive log ratio transform.

  This function transforms compositions from the non-isometric real space of alrs to Aitchison geometry.

  \(alr^{-1}: \mathbb{R}^{D-1} \rightarrow S^D\)

  The inverse alr transformation is defined as follows

  \[alr^{-1}(x) = C[exp([y_1, y_2, ..., y_{D-1}, 0])]\]

  where \(C[x]\) is the closure operation defined as

  \[C[x] = \left[\frac{x_1}{\sum_{i=1}^{D} x_i},\ldots, \frac{x_D}{\sum_{i=1}^{D} x_i} \right]\]

  for some \(D\) dimensional real vector \(x\) and \(D\) is the number of components for every composition.

  Parameters:

  - mat (numpy.ndarray) – a matrix of alr-transformed data
  - denominator_idx (int) – the index of the column (2D-composition) or position (1D-composition) of the output where the common denominator should be placed. By default denominator_idx=0 to specify the first column or position.

  Returns: | Inverse alr transformed matrix or vector where rows sum to 1.
  Return type: | numpy.ndarray

  Examples

      >>> import numpy as np
      >>> from skbio.stats.composition import alr, alr_inv
      >>> x = np.array([.1, .3, .4, .2])
      >>> alr_inv(alr(x))
      array([ 0.1,  0.3,  0.4,  0.2])