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A260901
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a(n) is the smallest composite k such that d(2)/d(1) + d(3)/d(2) + ... + d(q)/d(q-1) = prime(n), where d(1) < d(2) < ... < d(q) are the q divisors of k, or 0 if no such k exists.
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0
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0, 0, 0, 0, 50, 98, 108, 338, 578, 1058, 3072, 1922, 73167, 2738, 3362, 4418, 5618, 357735, 7442, 467985, 8978, 10658, 600285, 13778, 754635, 798735, 18818, 20402, 21218, 22898, 1085385, 2744445, 34322, 1217685, 1327935, 4548765, 45602, 49298, 1526385, 55778
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OFFSET
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1,5
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COMMENTS
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We observe two families of numbers having interesting properties:
(i) Integers having 6 divisors. These numbers are of the form m = 2*p^2 == 2 (mod 48) where p is prime and belongs to the set A = {5, 7, 13, 17, 23, 31, 37, 41, 47, 53, 61, 67, 73, 83, 97, 101, 103, 107, 131, 151, ...}. We find finite multiplicative groups A/qZ. For example, with q = 12 we obtain the group A/12Z = {1, 5, 7, 11}. From the first 5000 values of p, the finite groups are obtained with q = 2, 3, 4, 6, 8, 9, 12, 16, 18, 19, 24, 27, 29, 32, 36, 38, 43, 48, 54, 58, 59, 64, ...
(ii) Integers having more than 6 divisors. These numbers are 108, 3072, 73167, 357735, 467985, 600285, 1085385, ... with the corresponding numbers of divisors 12, 22, 8, 16, 16, 16, 16, 16, ...
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LINKS
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EXAMPLE
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a(7) = 108 because the divisors are {1, 2, 3, 4, 6, 9, 12, 18, 27, 36, 54, 108} => 2/1 + 3/2 + 4/3 + 6/4 + 9/6 + 12/9 + 18/12 + 27/18 + 36/27 + 54/36 + 108/54 = 17 = prime(7).
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MAPLE
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with(numtheory): nn:=36:
for n from 5 to nn do:
ii:=0:
for k from 1 to 4*10^6 while(ii=0) do:
d:=divisors(k):n0:=nops(d):
s1:=sum(‘d[i+1]/d[i]’, ‘i’=1..n0-1):
if type(k, prime)=false and s1=ithprime(n)
then
ii:=1:printf ( “%d %d \n”, n, k):
else
fi:
od:
od:
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PROG
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(PARI) a(n) = {m = 2; pn = prime(n); while ((d = divisors(m)) && (sum(k=2, #d, d[k]/d[k-1]) != pn), m++; if (isprime(m), m++)); m; } \\ Michel Marcus, Feb 14 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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