OFFSET
1,1
COMMENTS
Numbers of the form pq where p,q are odd primes, p<q and q is congruent to 1 mod p.
The corresponding non-Abelian groups are the semidirect products of Z/qZ and Z/pZ. - Bernard Schott, May 16 2020
LINKS
Jinyuan Wang, Table of n, a(n) for n = 1..1000
S. K. Berberian, Non-Abelian Groups of Order pq, American Mathematical Monthly, Vol. 60, No. 1, Jan. 1953, 37-40.
MATHEMATICA
Select[1 + 2*Range[500], (f = FactorInteger[#]; Last /@ f == {1, 1} && Mod @@ Reverse[First /@ f] == 1) &] (* Giovanni Resta, Apr 14 2013 *)
PROG
(PARI) lista(nn) = {forstep(n=1, nn, 2, my(f=factor(n)); if ((#f~ == 2) && (vecmax(f[, 2]) == 1) && ((f[2, 1] % f[1, 1]) == 1), print1(n, ", ")); ); } \\ Michel Marcus, Sep 28 2017
(PARI) list(lim)=my(v=List()); if(lim<9, return([])); forprime(p=3, sqrtint(((lim\=1)-1)\2), forprimestep(q=2*p+1, lim, 2*p, listput(v, p*q))); Set(v) \\ Charles R Greathouse IV, Feb 08 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
David Brown, Apr 14 2013
EXTENSIONS
More terms from Jinyuan Wang, May 16 2020
STATUS
approved