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A073904 Smallest multiple k*n of n having n divisors. 8
1, 2, 9, 8, 625, 12, 117649, 24, 36, 80, 25937424601, 60, 23298085122481, 448, 2025, 384, 48661191875666868481, 180, 104127350297911241532841, 240, 35721, 11264, 907846434775996175406740561329, 360, 10000, 53248, 26244, 1344 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) = n * A076931(n). - Thomas Ordowski, Oct 07 2005

Smallest refactorable number, m, such that m=k*n has n divisors. - Robert G. Wilson v, Oct 31 2005

LINKS

Table of n, a(n) for n=1..28.

FORMULA

If p is a prime then a(p) = p^(p-1). If n = p^2 then a(n) = 2^(p-1)*p^(p-1).

a(p^r) = (2*3*5*...*p_r)^(p-1) for r < p <= p_r. a(p^r) = (2*3*...*p_(r-1))^(p-1)*p^(p-1) for p > p_r. Else a(p^r) = ...? for r >= p. Problem a(2^r) = ...? Cf. A005179(p^n)=(2*3*...*p_n)^(p-1) for p_n < 2^p. - Thomas Ordowski, Aug 20 2005

Further comments from Thomas Ordowski, Aug 22 2005: a(p^r) = (2*3...*p_(r-1)*p)^(p-1) for p > p_r; else a(p^r) = (2*3...*p...*p_m)^(p-1)*p^(p^k-p) for p <= p_r and p_m < 2^p, where m=r-k+1 for smallest k such that p^k > r, so k=floor(log(r)/log(p))+1 and p > log(p_m)/log(2). Examples: If k=1 then a(p^r) = (2*3*...*p_r)^(p-1) for r < p <= p_r. If p=2 then a(2^r) = (2*3*...*p_m)*2^(2^k-2) for r < 5. For instance, let r=4 so k=3, m=2 and a(2^4)=384.

If p is a prime and n=p^r then a(p^r) = (s_1*s_2*...*s_r)^(p-1) where (s_r) is a permutation of the (ascending sequence) numbers of the form q^(p^j) for every prime q and j>=0; permutation such that s_(p^j)=p^(p^j) and shifted remainder. For example, if p=3 then (s_r): 3, 2, 3^3, 5, 7, 2^3, 11, 13, 3^9, 17, 19, ... so a(3^r) = (3*2*27*5*...*s_r)^2. - Thomas Ordowski, Aug 29 2005

If n=2^r then a(2^r) is the product of the first r members of the A109429 sequence. - Thomas Ordowski, Aug 29 2005

EXAMPLE

Smallest multiple a(n)=k*n; a(1)=1*1, a(2)=1*2, a(3)=3*3, a(4)=2*4, a(5)=125*5, a(6)=2*6, ... having d(k*n)=n divisors; d(1)=1, d(2)=2, d(3^2)=3, d(2^3)=4, d(5^4)=5, d(2^2*3)=3*2=6, ...

MATHEMATICA

f[n_] := Block[{k = 1}, If[ PrimeQ[n], n^(n - 1), While[d = DivisorSigma[0, k*n]; d != n, k++ ]; k*n]]; Table[ f[n], {n, 28}] (* Robert G. Wilson v *)

CROSSREFS

Cf. A076931, A050376, A005179, A037992, A050376, A111172.

Cf. A033950 (refactorable numbers, also known as tau numbers).

Cf. A110821 "SuperRefactorable Numbers".

Sequence in context: A324553 A230283 A121067 * A036879 A281389 A073927

Adjacent sequences:  A073901 A073902 A073903 * A073905 A073906 A073907

KEYWORD

nice,nonn

AUTHOR

Amarnath Murthy, Aug 18 2002

EXTENSIONS

a(12) corrected by Thomas Ordowski, Aug 18 2005.

Further corrections from Thomas Ordowski, Oct 07 2005

a(21), a(27) & a(28) from Robert G. Wilson v, Oct 31 2005

STATUS

approved

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Last modified November 27 12:51 EST 2021. Contains 349394 sequences. (Running on oeis4.)