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A252865 a(n) = n if n <= 3, otherwise the smallest squarefree number not occurring earlier having at least one common factor with a(n-2), but none with a(n-1). 4
1, 2, 3, 10, 21, 5, 6, 35, 22, 7, 11, 14, 33, 26, 15, 13, 30, 91, 34, 39, 17, 42, 85, 38, 51, 19, 66, 95, 46, 55, 23, 65, 69, 70, 57, 58, 93, 29, 31, 87, 62, 105, 74, 77, 37, 110, 111, 82, 129, 41, 43, 123, 86, 141, 106, 47, 53, 94, 159, 118, 165, 59, 78, 295 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Similar to A098550, but the restriction to squarefree makes it more a sequence of sets of primes, represented by their product.

The sequence has consecutive primes at indices 2 (2 & 3), 10 (7 & 11), 38 (29 & 31), 50 (41 & 43), and 56 (47 & 53). We conjecture that there are no further such pairs.

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

David L. Applegate, Hans Havermann, Bob Selcoe, Vladimir Shevelev, N. J. A. Sloane, and Reinhard Zumkeller, The Yellowstone Permutation, arXiv preprint arXiv:1501.01669, 2015.

PROG

(PARI) invecn(v, k, x)=for(i=1, k, if(v[i]==x, return(i))); 0

alist(n)=local(v=vector(n, i, i), x); for(k=4, n, x=4; while(!issquarefree(x)||invecn(v, k-1, x)||gcd(v[k-2], x)==1||gcd(v[k-1], x)!=1, x++); v[k]=x); v

(Haskell)

import Data.List (delete)

a252865 n = a252865_list !! (n-1)

a252865_list = 1 : 2 : 3 : f 2 3 (drop 3 a005117_list) where

   f u v ws = g ws where

     g (x:xs) = if gcd x u > 1 && gcd x v == 1

                   then x : f v x (delete x ws) else g xs

-- Reinhard Zumkeller, Dec 24 2014

(Python)

from fractions import gcd

from sympy import factorint

A252865_list, l1, l2, s, b = [1, 2, 3], 3, 2, 4, set()

for _ in range(10**2):

....i = s

....while True:

........if max(factorint(i).values()) == 1:

............if not i in b and gcd(i, l1) == 1 and gcd(i, l2) > 1:

................A252865_list.append(i)

................l2, l1 = l1, i

................b.add(i)

................while s in b:

....................b.remove(s)

....................s += 1

................break

........else:

............b.add(i)

........i += 1 # Chai Wah Wu, Dec 24 2014

CROSSREFS

Cf. A098550, A005117, A252867, A252868.

Cf. A251391.

Sequence in context: A089791 A226356 A141050 * A252868 A225477 A079161

Adjacent sequences:  A252862 A252863 A252864 * A252866 A252867 A252868

KEYWORD

nonn

AUTHOR

Franklin T. Adams-Watters, Dec 23 2014

STATUS

approved

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Last modified April 23 18:45 EDT 2017. Contains 285329 sequences.