OFFSET
1,1
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
Wikipedia, Sphenic number
FORMULA
a(n) = A066509(n)+1.
EXAMPLE
1309, 1310 and 1311 factor as 7*11*17, 2*5*131 and 3*19*23, respectively. No smaller such trio exists, so a(1)=1310.
MATHEMATICA
a248202[n_Integer] := Select[Range[n],
And[And[PrimeNu[#] == 3, PrimeNu[# - 1] == 3, PrimeNu[# + 1] == 3], And[PrimeOmega[#] == 3, PrimeOmega[# - 1] == 3, PrimeOmega[# + 1] == 3]] &]; a248202[20166](* Michael De Vlieger, Nov 06 2014 *)
f[n_]:=Last/@FactorInteger[n]=={1, 1, 1}; lst={}; Do[If[f[n]&&f[n+1]&&f[n+2], AppendTo[lst, n + 1]], {n, 17000}]; lst (* Vincenzo Librandi, Jul 24 2015 *)
PROG
(PARI) sq(n)=bigomega(n)==3 && omega(n)==3;
for(n=3, 10^5, if(sq(n-1)&&sq(n)&&sq(n+1), print1(n, ", ")));
\\ Joerg Arndt, Oct 18 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
James G. Merickel, Oct 03 2014
STATUS
approved