AR Coefficient Sum Stationarity Check

Problem

Use a simple sufficient stationarity check for an AR(p) process: the sum of the absolute values of the autoregressive coefficients must be strictly less than one.

Return True for an empty coefficient list, which represents white noise.

Hint

The condition is sufficient but not necessary for stationarity.

Test cases

case 1

in: [[0.5,0.3]]

expected: true

case 2

in: [[0.8,0.4]]

expected: false

case 3

in: [[]]

expected: true

case 4

in: [[-0.5,0.25]]

expected: true

case 5

in: [[1]]

expected: false

Starter codePython

def ar_sum_stationary(phis):
    # Your code here
    pass

5 test cases · run your code against them in the editor