

A089913


Table T(n,k) = lcm(n,k)/gcd(n,k) = n*k/gcd(n,k)^2 read by antidiagonals (n >= 1, k >= 1).


6



1, 2, 2, 3, 1, 3, 4, 6, 6, 4, 5, 2, 1, 2, 5, 6, 10, 12, 12, 10, 6, 7, 3, 15, 1, 15, 3, 7, 8, 14, 2, 20, 20, 2, 14, 8, 9, 4, 21, 6, 1, 6, 21, 4, 9, 10, 18, 24, 28, 30, 30, 28, 24, 18, 10, 11, 5, 3, 2, 35, 1, 35, 2, 3, 5, 11, 12, 22, 30, 36, 40, 42, 42, 40, 36, 30, 22, 12, 13, 6, 33, 10, 45
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OFFSET

1,2


COMMENTS

A multiplicative analog of absolute difference A049581. Exponents in prime factorization of T(n,k) are absolute differences of those of n and k. Commutative nonassociative operator with identity 1. T(nx,kx)=T(n,k), T(n^x,k^x)=T(n,k)^x, etc.
The bivariate function log(T(., .)) is a distance (or metric) function. It is a weighted analog of A130836, in the sense that if e_i (resp. f_i) denotes the exponent of prime p_i in the factorization of m (resp. of n), then both log(T(m, n)) and A130836(m, n) are writable as Sum_{i} w_i * abs(e_i  f_i). For A130836, w_i = 1 for all i, whereas for log(T(., .)), w_i = log(p_i).  Luc Rousseau, Sep 17 2018
If the analog of absolute difference, as described in the first comment, is determined by factorization into distinct terms of A050376 instead of by prime factorization, the equivalent operation is defined by A059897 and is associative. The positive integers form a group under A059897. The two factorization methods give the same factorization for squarefree numbers (A005117), so that T(.,.) restricted to A005117 is associative. Thus the squarefree numbers likewise form a group under the operation defined by this sequence.  Peter Munn, Apr 04 2019


LINKS

Ivan Neretin, Table of n, a(n) for n = 1..5050


FORMULA

A130836(n, k) = A001222(T(n, k)).  Luc Rousseau, Sep 17 2018


EXAMPLE

T(6,10) = lcm(6,10)/gcd(6,10) = 30/2 = 15.
1, 2, 3, 4, 5, ...
2, 1, 6, 2, 10, ...
3, 6, 1, 12, 15, ...
4, 2, 12, 1, 20, ...
5, 10, 15, 20, 1, ...
...


MATHEMATICA

Flatten[Table[LCM[i, m  i]/GCD[i, m  i], {m, 15}, {i, m  1}]] (* Ivan Neretin, Apr 27 2015 *)


PROG

(GAP) T:=Flat(List([1..13], n>List([1..n1], k>Lcm(k, nk)/Gcd(k, nk)))); # Muniru A Asiru, Oct 24 2018
(PARI) A089913(n, k)=n*k/gcd(n, k)^2 \\ M. F. Hasler, Dec 06 2019


CROSSREFS

Cf. A049581, A003990, A003991, A130836, A001222, A005117, A050376, A059897.
Sequence in context: A227287 A289236 A280172 * A257522 A059897 A325821
Adjacent sequences: A089910 A089911 A089912 * A089914 A089915 A089916


KEYWORD

easy,nonn,tabl,changed


AUTHOR

Marc LeBrun, Nov 14 2003


STATUS

approved



