This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A291192 Squarefree composite numbers n such that n*sigma(n) is of the form k*(k+1). 1
 6, 42, 1430, 3686, 5685, 23815, 60235, 129778, 370991, 652289, 654545, 660265, 795405, 801645, 1532170, 3413267, 3457597, 4235270, 4282330, 8107937, 9679187, 10835013, 15464685, 15963578, 16636503, 24976497, 28122458, 29595310, 34759879, 35642479, 58525286 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also squarefree composite numbers n such that Product_{p|n, p prime} A002378(p) is in A002378. Is this sequence infinite? LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..63 EXAMPLE 42 = 2*3*7 is a term because 42*sigma(42) = 42(2+1)(3+1)(7+1) = 4032 = 63*64. PROG (PARI) is(n, f=factor(n))=#f~>1 && vecmax(f[, 2])==1 && ispolygonal(n*sigma(f)/2, 3) list(lim)=my(v=List()); forfactored(n=6, lim\1, if(call(is, n), listput(v, n[1]))); Vec(v) \\ Charles R Greathouse IV, Aug 22 2017 CROSSREFS Cf. A002378, A064987. Sequence in context: A074651 A273922 A139223 * A185222 A179861 A083940 Adjacent sequences:  A291189 A291190 A291191 * A291193 A291194 A291195 KEYWORD nonn AUTHOR Altug Alkan and Thomas Ordowski, Aug 20 2017 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 October 20 02:18 EDT 2019. Contains 328244 sequences. (Running on oeis4.)