OFFSET
1,1
COMMENTS
14^2-1=195=3*5*13, 16^2-1=255=3*5*17, 20^2-1=399=3*7*19.
All terms are even since n^2-1 for n odd is a multiple of 4. If m is a term, then (m-1, m+1) contains one prime and one nonsquare semiprime. - Chai Wah Wu, Mar 28 2016
LINKS
Chai Wah Wu, Table of n, a(n) for n = 1..10000
MATHEMATICA
Select[Range[6! ], Last/@FactorInteger[ #^2-1]=={1, 1, 1}&]
Sqrt[#+1]&/@Select[Sort[Times@@@Subsets[Prime[Range[100]], {3}]], IntegerQ[ Sqrt[#+1]]&] (* Harvey P. Dale, Jul 24 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Joseph Stephan Orlovsky, Apr 23 2010
STATUS
approved