OFFSET
1,1
LINKS
Muniru A Asiru, Table of n, a(n) for n = 1..10000
FORMULA
a(n)*(a(n)+1)/2 has exactly 10 prime factors.
{ m : A069904(m) = 10 }. - Alois P. Heinz, Aug 05 2019
EXAMPLE
a(1) = 255 because that is the smallest index of a triangular number which is also a 10-almost prime; specifically T(255) = 255*(255+1)/2 = 32640 = 2^7 * 3 * 5 * 17.
MATHEMATICA
BigOmega[n_Integer]:=Plus@@Last[Transpose[FactorInteger[n]]]; Do[ t=n*(n+1)/2; If[BigOmega[t]==10, Print[n, " ", t]; ]; , {n, 2, 5000}]; (* Ray Chandler, Dec 14 2004 *)
Flatten[Position[Accumulate[Range[5000]], _?(PrimeOmega[#]==10&)]] (* Harvey P. Dale, May 12 2011 *)
PROG
(Magma) [n: n in [2..4500] | &+[d[2]: d in Factorization((n*(n+1)))] eq 11]; // Vincenzo Librandi, Dec 22 2018
(GAP) F:=List([1..4300], n->Length(Factors(n*(n+1)/2)));; a:=Filtered([1..Length(F)], i->F[i]=10); # Muniru A Asiru, Dec 22 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Dec 14 2004
EXTENSIONS
More terms from Ray Chandler, Dec 14 2004
STATUS
approved