def _erf_inv_rule(primals_in, series_in):
x, = primals_in
series, = series_in
u = [x] + series
primal_out = lax.erf_inv(x)
v = [primal_out] + [None] * len(series)
# derivative on co-domain for caching purposes
deriv_const = np.sqrt(np.pi) / 2.
deriv_y = lambda y: lax.mul(deriv_const, lax.exp(lax.square(y)))
# manually propagate through deriv_y since we don't have lazy evaluation of sensitivities
c = [deriv_y(primal_out)] + [None] * (len(series) - 1)
tmp_sq = [lax.square(v[0])] + [None] * (len(series) - 1)
tmp_exp = [lax.exp(tmp_sq[0])] + [None] * (len(series) - 1)
for k in range(1, len(series)):
# we know c[:k], we compute c[k]
# propagate c to get v
v[k] = fact(k - 1) * sum(
_scale(k, j) * c[k - j] * u[j] for j in range(1, k + 1))
# propagate v to get next c
# square
tmp_sq[k] = fact(k) * sum(
_scale2(k, j) * v[k - j] * v[j] for j in range(k + 1))
# exp
tmp_exp[k] = fact(k - 1) * sum(
_scale(k, j) * tmp_exp[k - j] * tmp_sq[j] for j in range(1, k + 1))
# const
c[k] = deriv_const * tmp_exp[k]
# we can't, and don't need, to compute c[k+1], just need to get the last v[k]
k = len(series)
v[k] = fact(k - 1) * sum(
_scale(k, j) * c[k - j] * u[j] for j in range(1, k + 1))
primal_out, *series_out = v
return primal_out, series_out
def erfinv(x):
x, = _promote_args_inexact("erfinv", x)
return lax.erf_inv(x)
