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 A289508 a(n) is the GCD of the indices j for which the j-th prime p_j divides n. 97
 0, 1, 2, 1, 3, 1, 4, 1, 2, 1, 5, 1, 6, 1, 1, 1, 7, 1, 8, 1, 2, 1, 9, 1, 3, 1, 2, 1, 10, 1, 11, 1, 1, 1, 1, 1, 12, 1, 2, 1, 13, 1, 14, 1, 1, 1, 15, 1, 4, 1, 1, 1, 16, 1, 1, 1, 2, 1, 17, 1, 18, 1, 2, 1, 3, 1, 19, 1, 1, 1, 20, 1, 21, 1, 1, 1, 1, 1, 22, 1, 2, 1, 23 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The number n = Product_j p_j can be regarded as an index for the multiset of all the j's, occurring with multiplicity corresponding to the highest power of p_j dividing n. Then a(n) is the gcd of the elements of this multiset. Compare A056239, where the same encoding for integer multisets('Heinz encoding') is used, but where A056239(n) is the sum, rather than the gcd, of the elements of the corresponding multiset (partition) of the j's. Cf. also A003963, for which A003963(n) is the product of the elements of the corresponding multiset. a(m*n) = gcd(a(m),a(n)). - Robert Israel, Jul 19 2017 LINKS Alois P. Heinz, Table of n, a(n) for n = 1..20000 FORMULA a(n) = gcd_j j, where p_j divides n. a(n) = A289506(n)/A289507(n). EXAMPLE a(n) = 1 for all even n as 2 = p_1. Also a(p_j) = j. Further, a(703) = 4 because 703 = p_8.p_{12} and gcd(8,12) = 4. MAPLE f:= n -> igcd(op(map(numtheory:-pi, numtheory:-factorset(n)))): map(f, [\$1..100]); # Robert Israel, Jul 19 2017 MATHEMATICA Table[GCD @@ Map[PrimePi, FactorInteger[n][[All, 1]] ], {n, 2, 83}] (* Michael De Vlieger, Jul 19 2017 *) PROG (PARI) a(n) = my(f=factor(n)); gcd(apply(x->primepi(x), f[, 1])); \\ Michel Marcus, Jul 19 2017 (Python) from sympy import primefactors, primepi, gcd def a(n): return gcd([primepi(d) for d in primefactors(n)]) print([a(n) for n in range(2, 101)]) # Indranil Ghosh, Jul 20 2017 CROSSREFS Cf. A289506, A289507. Sequence in context: A280504 A087267 A128267 * A028920 A260738 A055396 Adjacent sequences: A289505 A289506 A289507 * A289509 A289510 A289511 KEYWORD easy,nonn AUTHOR Christopher J. Smyth, Jul 11 2017 STATUS approved

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Last modified December 9 23:05 EST 2022. Contains 358710 sequences. (Running on oeis4.)