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

 

Logo

Annual Appeal: Today, Nov 11 2014, is the 4th anniversary of the launch of the new OEIS web site. 70,000 sequences have been added in these four years, all edited by volunteers. Please make a donation (tax deductible in the US) to help keep the OEIS running.

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 November 27 14:16 EST 2014. Contains 250210 sequences.