login
A085722
Numbers k such that k^2 + 1 is a semiprime.
40
3, 5, 8, 9, 11, 12, 15, 19, 22, 25, 28, 29, 30, 34, 35, 39, 42, 44, 45, 46, 48, 49, 50, 51, 52, 58, 59, 60, 61, 62, 64, 65, 69, 71, 76, 78, 79, 80, 85, 86, 88, 92, 95, 96, 100, 101, 102, 104, 106, 108, 114, 121, 131, 136, 139, 140, 141, 144, 145, 152, 154, 158, 159, 164
OFFSET
1,1
COMMENTS
Corresponding semiprimes k^2+1 are in A144255.
Solutions to the equation: A000005(1+k^2) = 4. - Enrique Pérez Herrero, May 03 2012
FORMULA
A085722 = A193432^-1({2}). - M. F. Hasler, Mar 11 2012
MATHEMATICA
lst={}; Do[If[Plus@@Last/@FactorInteger[n^2+1]==2, AppendTo[lst, n]], {n, 0, 200}]; lst (* Vladimir Joseph Stephan Orlovsky, Mar 24 2009 *)
Select[Range[200], PrimeOmega[#^2+1]==2&] (* Harvey P. Dale, Feb 28 2013 *)
PROG
(PARI) select(vector(50, n, n), n->bigomega(n^2+1)==2)
\\ Zak Seidov, Feb 25 2011
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jason Earls, Jul 20 2003
STATUS
approved