modality N1 rare writtenliterary
に難くない — not hard to ~
に難くない ・ にかたくない
A full written guide for に難くない is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See に難くない in real sentences
Jengo shows に難くない the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Advanced Japanese Grammar.