login
This site is supported by donations to The OEIS Foundation.

 

Logo

The October issue of the Notices of the Amer. Math. Soc. has an article about the OEIS.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A065302 Squarefree composite numbers whose sum of divisors is also squarefree. 2
1, 26, 74, 122, 146, 218, 314, 362, 386, 458, 554, 626, 746, 794, 818, 842, 866, 914, 1082, 1202, 1226, 1322, 1346, 1418, 1466, 1514, 1538, 1658, 1706, 1754, 1874, 1994, 2018, 2042, 2066, 2138, 2186, 2234, 2258, 2306, 2402, 2426, 2474, 2594, 2642, 2762 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

All elements except the first, a(1)=1, are of the form 2*p, where p is a prime and p == 1 (mod 12). Also, sigma(2*p) = (1+2)*(1+p) = 6m where m = (1+p)/2 and m == 1 (mod 6). A squarefree composite number not of the form 2*p cannot be in the sequence since sigma is multiplicative. For example, sigma(p*q) = (1+p)*(1+q) is divisible by 4 for p,q > 2. - Walter Kehowski, Mar 21 2007

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..1000

PROG

(PARI) { n=0; for (m = 1, 10^9, if (!isprime(m) && moebius(m) && moebius(sigma(m)), write("b065302.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Oct 16 2009

CROSSREFS

Cf. A000203, A008683, A065299-A065303.

Cf. A128607, A128608.

Sequence in context: A039377 A043200 A043980 * A044164 A044545 A255185

Adjacent sequences:  A065299 A065300 A065301 * A065303 A065304 A065305

KEYWORD

nonn

AUTHOR

Labos Elemer, Oct 29 2001

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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 24 17:44 EDT 2018. Contains 315347 sequences. (Running on oeis4.)