HJ Plummer
HJ may refer to: Science, technology, and mathematics * Hall–Janko group, a mathematical group * U.S. code for a cryptographic key change; see cryptoperiod Other uses * , a two-letter combination used in some languages ** /hj/, a pronunciation cluster, sometimes reduced * Holden HJ, an Australian car 1974-1976 * Hajji (Hj.), an Islamic honorific * ''hic jacet'' ('here lies'), Latin phrase on gravestones * Hilal-i-Jurat, post-nominal for Pakistan honour * Hitler-Jugend (Hitler Youth) * Tasman Cargo Airlines, IATA airline designator * Handjob A handjob, also spelled hand job, is a sex act, performed as either foreplay or as non-penetrative sex, that involves the manual stimulation of the penis or scrotum by another person to induce an erection for sexual pleasure, sexual arousal a ...

Hall–Janko Group
In the area of modern algebra known as group theory, the Janko group ''J2'' or the Hall-Janko group ''HJ'' is a sporadic simple group of Order (group theory), order :   2733527 = 604800 : ≈ 6. History and properties ''J2'' is one of the 26 Sporadic groups and is also called Hall–Janko–Wales group. In 1969 Zvonimir Janko predicted J2 as one of two new simple groups having 21+4:A5 as a centralizer of an involution (the other is the Janko group J3). It was constructed by as a rank 3 permutation group on 100 points. Both the Schur multiplier and the outer automorphism group have order 2. As a permutation group on 100 points J2 has involution (mathematics)#group theory, involutions moving all 100 points and involutions moving just 80 points. The former involutions are products of 25 double transportions, an odd number, and hence lift to 4-elements in the Covering_groups_of_the_alternating_and_symmetric_groups, double cover 2.A100. The double cover 2.J2 occur ...
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