The OEIS is supported by the many generous donors to the OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A307958 Coreful perfect numbers: numbers k such that csigma(k) = 2*k, where csigma(k) is the sum of the coreful divisors of k (A057723). 15
 36, 180, 252, 392, 396, 468, 612, 684, 828, 1044, 1116, 1176, 1260, 1332, 1476, 1548, 1692, 1908, 1960, 1980, 2124, 2196, 2340, 2412, 2556, 2628, 2772, 2844, 2988, 3060, 3204, 3276, 3420, 3492, 3636, 3708, 3852, 3924, 4068, 4140, 4284, 4312, 4572, 4716, 4788 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Hardy and Subbarao defined a coreful divisor d of a number k as a divisor with the same set of distinct prime factors as k, or rad(d) = rad(k), where rad(k) is the largest squarefree divisor of k (A007947). The number of these divisors is A005361(k) and their sum is csigma(k) = A057723(k). Since csigma(k) is multiplicative and csigma(p) = p for prime p, then if k is coreful perfect number, then also m*k is, for any coprime m, gcd(m, k) = 1. Thus there are infinitely many coreful perfect numbers, and all of them can be generated from the sequence of primitive coreful perfect numbers (A307959), which is the subsequence of powerful terms of this sequence. This sequence and A307959 are analogous to e-perfect numbers (A054979) and primitive e-perfect numbers (A054980). LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 G. E. Hardy and M. V. Subbarao, Highly powerful numbers, Congress. Numer., Vol. 37 (1983), pp. 277-307. (Annotated scanned copy) EXAMPLE 36 is in the sequence since its coreful divisors are 6, 12, 18, 36, whose sum is 72 = 2 * 36. MATHEMATICA f[p_, e_] := (p^(e+1)-1)/(p-1)-1; a=1; a[n_] := Times @@ (f @@@ FactorInteger[n]); s={}; Do[If[a[n] == 2n, AppendTo[s, n]], {n, 1, 10^6}]; s PROG (PARI) rad(n) = factorback(factorint(n)[, 1]); \\ A007947 s(n) = rad(n)*sigma(n/rad(n)); \\ A057723 isok(n) = s(n) == 2*n; \\ Michel Marcus, May 14 2019 CROSSREFS Cf. A005361, A007947, A054979, A054980, A057723, A307959, A307960, A307961. Sequence in context: A211751 A211762 A219858 * A348962 A127657 A318100 Adjacent sequences: A307955 A307956 A307957 * A307959 A307960 A307961 KEYWORD nonn AUTHOR Amiram Eldar, May 08 2019 STATUS approved

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

Last modified January 27 17:42 EST 2023. Contains 359845 sequences. (Running on oeis4.)