The OEIS is supported by the many generous donors to the OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A128905 Numbers k such that the k-th triangular number has exactly four distinct prime factors. 2
 20, 51, 59, 60, 65, 68, 69, 76, 77, 83, 91, 92, 105, 110, 114, 115, 123, 129, 131, 139, 154, 156, 165, 182, 185, 186, 187, 194, 210, 212, 221, 227, 228, 235, 236, 237, 246, 254, 258, 265, 266, 267, 273, 276, 286, 290, 291, 307, 309, 318, 321, 322, 330, 345 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Or, indices of triangular numbers with exactly four distinct prime factors. LINKS FORMULA a(n)=k and T(k)=k(k+1)/2=p*q*r*s for some k, p, q, r, s where T(k) is a triangular number and p, q, r, s are distinct primes. EXAMPLE In order of increasing p (the least prime factor of T(k)):   a(1)  =  20 because T(20)  =    210 =  2* 3* 5* 7,   a(5)  =  65 because T(65)  =   2145 =  3* 5*11*13,   a(21) = 154 because T(154) =  11935 =  5* 7*11*31,   a(45) = 286 because T(286) =  41041 =  7*11*13*41,   a(143)= 781 because T(781) = 305371 = 11*17*23*71,   a(91) = 493 because T(493) = 121771 = 13*17*19*29, etc. CROSSREFS Cf. A000217, A068443, A069903, A076551, A127637, A128896. Sequence in context: A345124 A049390 A220040 * A276135 A350115 A211143 Adjacent sequences:  A128902 A128903 A128904 * A128906 A128907 A128908 KEYWORD nonn AUTHOR Zak Seidov, Apr 22 2007 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified January 28 11:01 EST 2022. Contains 350655 sequences. (Running on oeis4.)