OFFSET
1,1
COMMENTS
If n is the m-th composite, then a(n) = A143772(m).
If n is prime, then a(n) is defined as n+1, since a(n) = gcd(1+n, n+1).
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..16384 (terms 2..10000 from Michael De Vlieger)
EXAMPLE
a(1) = gcd(1+1) = 2, i.e., the greatest common divisor of a singular set [2].
a(9) = gcd(1+9, 3+3, 9+1) = 2.
a(20) = gcd(1+20, 2+10, 4+5, 5+4, 10+2, 20+1) = 3.
a(44) = gcd(1+44, 2+22, 4+11, 11+4, 22+2, 44+1) = 3.
MAPLE
A143771 := proc(n) local dvs ; dvs := convert(numtheory[divisors](n), list) ; igcd(seq( op(i, dvs)+n/op(i, dvs), i=1..nops(dvs))) ; end: for n from 2 to 140 do printf("%d, ", A143771(n)) ; od: # R. J. Mathar, Sep 05 2008
MATHEMATICA
Table[GCD @@ Map[# + n/# &, Divisors@ n], {n, 2, 96}] (* Michael De Vlieger, Oct 30 2017 *)
PROG
(PARI) a(n) = my(d = divisors(n)); gcd(vector(#d, k, d[k]+n/d[k])); \\ Michel Marcus, Oct 05 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Aug 31 2008
EXTENSIONS
Extended by R. J. Mathar, Sep 05 2008
Term a(1) = 2 prepended and Example-section extended by Antti Karttunen, Mar 29 2021
STATUS
approved