OFFSET
1,3
COMMENTS
Conjecture: There are finitely many numbers such that a(n)is not == 0 (mod a(n-1). (Also mentioned in A093455.)
Product of all composite numbers between n*(n-1)/2+1 and n*(n+1)/2 (including boundaries). - Stefan Steinerberger, Apr 02 2006
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..244
EXAMPLE
a(5) = 12*14*15 = 2520.
MATHEMATICA
Table[a := Range[n*(n - 1)/2 + 1, n*(n + 1)/2]; b := Select[a, Not[PrimeQ[ # ]] &]; Product[b[[i]], {i, 1, Length[b]}], {n, 1, 20}] (* Stefan Steinerberger, Apr 02 2006 *)
Module[{nn=20}, Times@@Select[#, CompositeQ]&/@TakeList[Range[(nn(nn+1))/2], Range[nn]]] (* Harvey P. Dale, Dec 30 2024 *)
CROSSREFS
KEYWORD
nonn,less,changed
AUTHOR
Amarnath Murthy, Apr 03 2004
EXTENSIONS
More terms from Stefan Steinerberger, Apr 02 2006
STATUS
approved