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 A212180 Number of distinct second signatures (cf. A212172) represented among divisors of n. 6

%I

%S 1,1,1,2,1,1,1,3,2,1,1,2,1,1,1,4,1,2,1,2,1,1,1,3,2,1,3,2,1,1,1,5,1,1,

%T 1,3,1,1,1,3,1,1,1,2,2,1,1,4,2,2,1,2,1,3,1,3,1,1,1,2,1,1,2,6,1,1,1,2,

%U 1,1,1,5,1,1,2,2,1,1,1,4,4,1,1,2,1,1,1,3

%N Number of distinct second signatures (cf. A212172) represented among divisors of n.

%C Completely determined by the exponents >=2 in the prime factorization of n (cf. A212172, A212173).

%C The fraction of the divisors of n which have a given second signature {S} is also a function of n's second signature. For example, if n has second signature {3,2}, it follows that 1/3 of n's divisors are squarefree. Squarefree numbers are represented with 0s in A212172, in accord with the usual OEIS custom of using 0 for nonexistent elements; in comments, their second signature is represented as { }.

%H Antti Karttunen, <a href="/A212180/b212180.txt">Table of n, a(n) for n = 1..10000</a>

%H <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>

%e The divisors of 72 represent a total of 5 distinct second signatures (cf. A212172), as can be seen from the exponents >= 2, if any, in the canonical prime factorization of each divisor:

%e { }: 1, 2 (prime), 3 (prime), 6 (2*3)

%e {2}: 4 (2^2), 9 (3^2), 12 (2^2*3), 18 (2*3^2)

%e {3}: 8 (2^3), 24 (2^3*3)

%e {2,2}: 36 (2^2*3^2)

%e {3,2}: 72 (2^3*3^2)

%e Hence, a(72) = 5.

%t Array[Length@ Union@ Map[Sort@ Select[FactorInteger[#][[All, -1]], # >= 2 &] &, Divisors@ #] &, 88] (* _Michael De Vlieger_, Jul 19 2017 *)

%o (PARI)

%o A046523(n) = { my(f=vecsort(factor(n)[, 2], , 4), p); prod(i=1, #f, (p=nextprime(p+1))^f[i]); }; \\ This function from _Charles R Greathouse IV_, Aug 17 2011

%o A057521(n) = { my(f=factor(n)); prod(i=1, #f~, if(f[i, 2]>1, f[i, 1]^f[i, 2], 1)); } \\ This function from _Charles R Greathouse IV_, Aug 13 2013

%o A212173(n) = A046523(A057521(n));

%o A212180(n) = { my(vals = Set()); fordiv(n, d, vals = Set(concat(vals, A212173(d)))); length(vals); }; \\ _Antti Karttunen_, Jul 19 2017

%o (Python)

%o from sympy import factorint, divisors

%o from operator import mul

%o def P(n): return sorted(factorint(n).values())

%o def a046523(n):

%o x=1

%o while True:

%o if P(n)==P(x): return x

%o else: x+=1

%o def a057521(n): return 1 if n==1 else reduce(mul, [1 if e==1 else p**e for p, e in factorint(n).items()])

%o def a212173(n): return a046523(a057521(n))

%o def a(n):

%o l=[]

%o for d in divisors(n):

%o x=a212173(d)

%o if not x in l:l+=[x, ]

%o return len(l)

%o print map(a, xrange(1, 151)) # _Indranil Ghosh_, Jul 19 2017

%Y Cf. A212172, A085082, A088873, A181796, A182860, A212173, A212642, A212643, A212644.

%K nonn

%O 1,4

%A _Matthew Vandermast_, Jun 04 2012

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Last modified December 6 18:03 EST 2019. Contains 329809 sequences. (Running on oeis4.)