Compute "deserium" numbers in SQL

deserium.sql

CREATE OR REPLACE FUNCTION deserium(num INTEGER) RETURNS BOOLEAN AS $$

WITH RECURSIVE 
 digts(d, m, pos) AS (
    SELECT t / 10, t % 10, 1 FROM (VALUES(num)) v(t)
    UNION ALL
        SELECT d / 10, d % 10, pos + 1
        FROM digts WHERE d > 0
 ),
 to_sum(v) AS (
    SELECT m ^ (MAX(pos) over () - pos + 1) FROM digts
 )
SELECT sum(v) :: int = num
FROM  to_sum;

$$ LANGUAGE SQL IMMUTABLE;