OFFSET
1,2
COMMENTS
From Robert Israel, Apr 05 2020: (Start)
If A001358(n) = 2*p, then a(n) = (p+1)/2 if p == 3 (mod 4), or (3*p+1)/2 if p == 1 (mod 4).
If A001358(n) = 3*p with p > 3, then a(n) = (3*p+1)/4 if p == 1 (mod 4), or (9*p+1)/4 if p == 3 (mod 4). (End)
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1)=1 because the first semiprime is 4=2*2 and 2^(2-2) mod 4 = 1.
a(11)=25 because the 11th semiprime is 33=3*11 and 2^(11-3) mod 33 = 25.
MAPLE
b:= proc(n) option remember;
local k;
if n=1 then 4
else for k from b(n-1)+1 while
isprime(k) or add (i[2], i=ifactors(k)[2])<>2
do od; k
fi
end:
a:= proc(n)
local l;
l:= ifactors (b(n))[2];
if nops (l)=1 then 1
else 2 &^ abs(l[1][1]-l[2][1]) mod b(n)
fi
end:
seq (a(n), n=1..65);
MATHEMATICA
Mod[2^Differences[FactorInteger[#][[All, 1]]], #]&/@Select[Range[300], PrimeOmega[ #] == 2&]/.{}->1//Flatten (* Harvey P. Dale, Dec 25 2018 *)
CROSSREFS
KEYWORD
nonn,look
AUTHOR
Juri-Stepan Gerasimov, Dec 16 2010
EXTENSIONS
Edited by Alois P. Heinz, Dec 17 2010
STATUS
approved