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A078502 a(n) = smallest positive integer N such that (N - k)/k is prime for k = 1, 2, ..., n. 2
3, 6, 12, 12, 174600, 7224840, 10780560, 10780560, 1086338816640, 50060257410240, 7720634052774720, 227457297898150320, 7272877497848202240, 7272877497848202240 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The idea for the sequence and first eleven terms are from Ken Wilke.

a(n) == 0 (mod 120) for n > 4: because a(n)/2, a(n)/3, a(n)/4 and a(n)/5 must be integer, a(n) == 0 (mod 60); and if a(n) == 60 (mod 120), (a(n)-4)/4 == 14 (mod 120) would not be prime; thus a(n) == 0 (mod 120). A more general result is a(n) == 0 (mod lcm(1,2,...,n)) for all n >= 1, and a(n) == 0 (mod 2*lcm(1,2,...,n)) for n > 4. - Jean-Christophe Hervé, Sep 15 2014

LINKS

Table of n, a(n) for n=1..14.

C. Rivera, Puzzle 206. (N-k)/k Primes, Prime Puzzles and Problems Connection.

FORMULA

a(n) == 0 (mod A003418(n)) because of the divisibility condition (A003418(n) = lcm(1,2,...,n)). - Jean-Christophe Hervé, Sep 15 2014

EXAMPLE

(12-k)/k is prime for k = 1,2,3,4 and 12 is the smallest positive integer satisfying this property. Hence a(4) = 12.

PROG

(PARI)

a(n)=k=1; while(k, c=0; for(i=1, n, if(k%i==0&&isprime(k/i-1), c++)); if(c==n, return(k)); k++)

n=1; while(n<10, print1(a(n), ", "); n++) \\ Derek Orr, Sep 15 2014

CROSSREFS

See A093554 for another version.

Cf. A074200 (equivalent sequence for (N+k)/k prime).

Sequence in context: A182633 A038587 A090553 * A107974 A298879 A116625

Adjacent sequences:  A078499 A078500 A078501 * A078503 A078504 A078505

KEYWORD

nonn,more

AUTHOR

Joseph L. Pe, Jan 05 2003

EXTENSIONS

Corrected and extended by Jens Kruse Andersen, Jan 10 2003

STATUS

approved

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Last modified May 22 06:32 EDT 2019. Contains 323478 sequences. (Running on oeis4.)