I have faced some cases with -적인 suffix used for forming adjectives instead of -적. How the -인 ending affects? Is there any difference between 력사적 and 력사적인, 천재적 and 천재적인, or 정력적 and 정력적인?
noun + 적 : it has two usage : noun and adnominal (or pre-noun)
1) For instance, 철학적 사고 = philosophical thought
It is adnominal whose meaning is being related with philosophy.
2) 철학적인 분석 = philosophical analysis.
It is a noun whose meaning is a thing related with philosophy. Here 철학적 (noun) + 인 = adjective.
@ He is philosophical 그는 철학적이다.
철학적인 그는 (O) 철학적 그는 (X) : In front of pronoun, we can not use 철학적(adnominal)