

A127724


kimperfect numbers for some k >= 1.


14



1, 2, 6, 12, 40, 120, 126, 252, 880, 2520, 2640, 10880, 30240, 32640, 37800, 37926, 55440, 75852, 685440, 758520, 831600, 2600640, 5533920, 6917400, 9102240, 10281600, 11377800, 16687440, 152182800, 206317440, 250311600, 475917120, 715816960, 866829600
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OFFSET

1,2


COMMENTS

For prime powers p^e, define a multiplicative function rho(p^e) = p^e  p^(e1) + p^(e2)  ... + (1)^e. A number n is called kimperfect if there is an integer k such that n = k*rho(n). Sequence A061020 gives a signed version of the rho function. As with multiperfect numbers (A007691), 2imperfect numbers are also called imperfect numbers. As shown by Iannucci, when rho(n) is prime, there is sometimes a technique for generating larger imperfect numbers.
Zhou and Zhu find 5 more terms, which are in the bfile.  T. D. Noe, Mar 31 2009
If a term t has a prime factor p from A065508 with exponent 1 and does not have the corresponding prime factor q from A074268, then t*p*q is also a term.  Michel Marcus, Nov 22 2017
For n >= 1, the least nimperfect numbers are 1, 2, 6, 993803899780063855042560.  Michel Marcus, Feb 13 2018
For any m > 0, if n*p^(2m1) is kimperfect, q = rho(p^(2m)) is prime and gcd(pq,n) = 1, then n*p^(2m)*q is also kimperfect.  M. F. Hasler, Feb 13 2020


REFERENCES

R. K. Guy, Unsolved Problems in Theory of Numbers, Springer, 1994, B1.


LINKS

Giovanni Resta, Table of n, a(n) for n = 1..50 (terms < 10^13, a(1)a(39) from T. D. Noe (from Iannucci, Zhou, and Zhu), a(40)a(44) from Donovan Johnson)
Weiyi Zhou and Long Zhu, On kimperfect numbers, INTEGERS: Electronic Journal of Combinatorial Number Theory, 9 (2009), #A01.


EXAMPLE

126 = 2*3^2*7, rho(126) = (21)*(93+1)*(71) = 42. 3*42 = 126, so 126 is 3imperfect.  Jud McCranie Sep 07 2019


MATHEMATICA

f[p_, e_]:=Sum[(1)^(ek) p^k, {k, 0, e}]; rho[n_]:=Times@@(f@@@FactorInteger[n]); Select[Range[10^6], Mod[ #, rho[ # ]]==0&]


PROG

(PARI) isok(n) = denominator(n/sumdiv(n, d, d*(1)^bigomega(n/d))) == 1; \\ Michel Marcus, Oct 28 2017
(PARI) upto(ulim) = {res = List([1]); rhomap = Map(); forprime(p = 2, 3, for(i = 1, logint(ulim, p), mapput(rhomap, p^i, rho(p^i)); iterate(p^i, mapget(rhomap, p^i), ulim))); listsort(res, 1); res}
iterate(m, rhoo, ulim) = {my(c); if(m / rhoo == m \ rhoo, listput(res, m); my(frho = factor(rhoo)); for(i = 1, #frho~, if(m%frho[i, 1] != 0, for(e = 1, logint(ulim \ m, frho[i, 1]), if(mapisdefined(rhomap, frho[i, 1]^e) == 0, mapput(rhomap, frho[i, 1]^e, rho(frho[i, 1]^e))); iterate(m * frho[i, 1]^e, rhoo * mapget(rhomap, frho[i, 1]^e), ulim)); next(2))))}
rho(n) = {my(f = factor(n), res = q = 1); for(i=1, #f~, q = 1; for(j = 1, f[i, 2], q = q + f[i, 1]^j); res * =q); res} \\ David A. Corneth, Nov 02 2017
/* It is known that the least odd term > 1 is > 10^49. This code defines an efficient function is_A127724, but A127724_vec is better computed with upto(.) */


CROSSREFS

Cf. A061020 (signed version of rho function), A206369 (the rho function).


KEYWORD

nice,nonn


AUTHOR



EXTENSIONS



STATUS

approved



