The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A190786 Numbers n such that sigma(2*n-1) = 3*n, where sigma(k) is the sum of the positive divisors of k. 0
 8, 104, 512, 1488, 9680, 73728, 603680, 2508800, 1085407232, 29473106432 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All even perfect numbers are of the form z(2*z-1) with z = 2^(p-1), p prime and 2*z-1 = 2^p-1 prime. It is unknown if there are any odd perfect numbers of this same form. The equation defining the sequence appears while working a special case of the conjecture. It is conjectured that all terms of this sequence are even numbers. a(11) > 5*10^11, according to Giovanni Resta at A063906. - Amiram Eldar, Jan 27 2019 LINKS FORMULA a(n) = (A063906(n)+1)/2. - Amiram Eldar, Jan 27 2019 EXAMPLE Example:  a(1)=8 since sigma(15)= 24 = 3*8. MATHEMATICA Select[Range[10^5], DivisorSigma[1, 2# - 1] == 3# &] (* Alonso del Arte, May 19 2011 *) PROG (PARI) zt(a, b) = {local(c, c1, c2, s); c =a ; c1 = 2*c-1; c2 = 3*c; while(c

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 27 18:06 EST 2022. Contains 350611 sequences. (Running on oeis4.)