If and only if

↔⇔≡
Logical symbols representing iff

In logic and related fields such as mathematics and philosophy, if and only if (shortened iff) is a biconditional logical connective between statements.

In that it is biconditional, the connective can be likened to the standard material conditional ("only if", equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other (i.e. either both statements are true, or both are false). It is controversial whether the connective thus defined is properly rendered by the English "if and only if", with its pre-existing meaning. There is nothing to stop one from stipulating that we may read this connective as "only if and if", although this may lead to confusion.

In writing, phrases commonly used, with debatable propriety, as alternatives to P "if and only if" Q include Q is necessary and sufficient for P, P is equivalent (or materially equivalent) to Q (compare material implication), P precisely if Q, P precisely (or exactly) when Q, P exactly in case Q, and P just in case Q. Many authors regard "iff" as unsuitable in formal writing; others use it freely.

JJ Lin

JJ Lin (traditional Chinese: 林俊傑; simplified Chinese: 林俊杰; pinyin: Lin Jun Jie) is a Singaporean singer, songwriter, record producer and actor based in Taiwan. JJ Lin was managed by Ocean Butterflies from 2003 to 2009. He is managed by Warner Music Taiwan. Lin won Best New Artist for his work on his debut album Music Voyager at the 15th Golden Melody Awards in 2004.He won Best Male Mandopop Performer for his work on his 10th album Stories Untold at the 25th Golden Melody Awards in 2014.

Biography

Lin was born in Singapore and studied at Anglo-Chinese School (Independent) and Saint Andrew's Junior College before completing his National Service with the Singapore Armed Forces.

Career

In addition to English and Mandarin, Lin sings in Hokkien and Cantonese using romanized pronunciation system, and has released his songs in Cantonese. In Taiwan, he was awarded the "Best New Artist" award in the Taiwan Golden Melody Awards.

He is well known for his songwriting skills. He has written songs for various musical artists while he was still a trainee under Ocean Butterflies. His notable compositions include "Remember" (記得 Jì Dé) for Taiwanese singer A-Mei, A-Do's "Let Go" (放手 Fàng Shǒu), Harlem Yu's "What's Wrong With You?" (幹嘛你看不爽我 Gàn Má Nǐ Kàn Bù Shuǎng Wǒ), Comic Boyz's "Heart of Superman" (超人心 Chāo Rén Xīn).