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

 

Logo

Many excellent designs for a new banner were submitted. We will use the best of them in rotation.

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A141705 a(n) = least Carmichael number of the form prime(n)*prime(n')*prime(n") with n < n' < n", or 0 if no such number exists. 2
0, 561, 1105, 1729, 0, 29341, 162401, 334153, 1615681, 3581761, 399001, 294409, 252601, 1152271, 104569501, 2508013, 178837201, 6189121, 10267951, 10024561, 14469841, 4461725581, 985052881, 19384289, 23382529, 3828001, 90698401 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Primes for which there are no such numbers (i.e. prime(n) such that a(n)=0) are given in A051663. Sequence A135720 is similar, but without restriction to 3-factor Carmichael numbers.

LINKS

Table of n, a(n) for n=1..27.

OEIS index entries for Carmichael numbers

EXAMPLE

a(1)=0 since there is no Carmichael number having prime(1)=2 as factor.

a(2)=561 since this is the smallest Carmichael number of the form pqr with prime r>q>p=prime(2)=3.

a(5)=0 since there is no Carmichael number of the form pqr with prime r>q>p=prime(5)=11.

PROG

(PARI) A141705(n) = { /* based on code by J.Brennen (jb AT brennen.net) */ local( V=[], B, p=prime(n), q, r); for( A=1, p-1, B=ceil((p^2+1)/A); while( 1, r=(p*B-p+A*B-B)/(A*B-p*p); q=(A*r-A+1)/p; q<=p && break; denominator(q)==1 && denominator(r)==1 && r>q && isprime(q) && isprime(r) && (p*q*r)%(p-1)==1 && V=concat(V, [p*q*r]); B++ )); if( V, vecmin( V )); }

CROSSREFS

Cf. A002997, A051663, A135720, A141702-A141706.

Sequence in context: A224695 A137198 A194231 * A135721 A047713 A006971

Adjacent sequences:  A141702 A141703 A141704 * A141706 A141707 A141708

KEYWORD

nonn

AUTHOR

M. F. Hasler, Jul 03 2008

STATUS

approved

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

Content is available under The OEIS End-User License Agreement .

Last modified April 23 22:11 EDT 2014. Contains 240947 sequences.