4 Jun
2011
4 Jun
'11
7:20 p.m.
[Fermat Half-Prime]: n such that 2|a|=|b|, for a in 1<a<n, a^(n-1)%n == 1, and 0<b<n relatively prime to n.
(Bug: it should be 0<a<n above.) Actually, this is somewhat interesting. The "Fermat Half-Primes" in 2..100000 are: 4,6,15,91,703,1891,2701,11305,12403,13981,18721,23001,30889,38503,39865,49141,68101,79003,88561,88831,91001,93961. OEIS lists has "A129521: Numbers of the form p*q, p and q prime with q=2*p-1", which appears to be a subset of this sequence: 6,15,91,703,1891,2701,12403,18721,38503,49141,79003,88831,104653,... (The Half-Primes not in A129521 are: 4,11305,13981,23001,30889,39865,68101,88561,91001,93961).