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 A069023 Define a subset of divisors of n to be a dedicated subset if the product of any two members is also a divisor of n. 1 is not allowed as a member as it gives trivially 1*d = d a divisor. a(n) is the number of dedicated subsets of divisors of n with at least two members. 1
 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 3, 0, 1, 1, 2, 0, 3, 0, 3, 1, 1, 0, 9, 0, 1, 1, 3, 0, 7, 0, 5, 1, 1, 1, 9, 0, 1, 1, 9, 0, 7, 0, 3, 3, 1, 0, 17, 0, 3, 1, 3, 0, 9, 1, 9, 1, 1, 0, 20, 0, 1, 3, 8, 1, 7, 0, 3, 1, 7, 0, 28, 0, 1, 3, 3, 1, 7, 0, 17, 2, 1, 0, 20, 1, 1, 1, 9, 0, 20, 1, 3, 1, 1, 1, 35, 0, 3, 3, 9, 0, 7 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,12 COMMENTS a(n) is determined by the prime signature of n. LINKS Antti Karttunen, Table of n, a(n) for n = 1..16384 Antti Karttunen, Scheme-program for computing this sequence FORMULA It seems that for n >= 3, a(p^n) = A077866(n-3). - Antti Karttunen, Nov 24 2017 EXAMPLE a(12) = 3. The divisors of 12 are 1,2,3,4,6,12. The divisor subsets (2,3),(2,6) and (3,4) are such that their product is also a divisor of 12. a(24) = 9 and the dedicated divisor subsets are (2,3),(2,4),(2,6),(2,12),(3,4),(3,8),(4,6),(2,3,4),(2,4,6). PROG (PARI) \\ The following program is very inefficient: A069023(n) = { if(bigomega(n)<2, return(0)); my(pds=(divisors(n)[2..numdiv(n)]), subsets = select(v -> (length(v)>=2), powerset(pds)), pair_products = apply(ss -> podp(ss), subsets), prodsmodn = apply(pps -> vector(#pps, i, n%pps[i]), pair_products)); length(select(s -> 0==vecsum(s), prodsmodn)); }; powerset(v) = { my(siz=2^length(v), pv=vector(siz)); for(i=0, siz-1, pv[i+1] = choosebybits(v, i)); pv; }; choosebybits(v, m) = { my(s=vector(hammingweight(m)), i=j=1); while(m>0, if(m%2, s[j] = v[i]; j++); i++; m >>= 1); s; }; podp(v) = { my(siz=binomial(length(v), 2), rv=vector(siz), k=0); for(i=1, length(v)-1, for(j=i+1, length(v), k++; rv[k] = v[i]*v[j])); rv; }; \\ podp = product of distinct pairs \\ Antti Karttunen, Nov 24 2017 (Scheme) ;; See in the links-section. CROSSREFS Cf. A077866. Sequence in context: A249442 A334739 A131802 * A275336 A091614 A249767 Adjacent sequences:  A069020 A069021 A069022 * A069024 A069025 A069026 KEYWORD nonn AUTHOR Amarnath Murthy, Apr 02 2002 EXTENSIONS Edited by David Wasserman, Mar 26 2003 STATUS approved

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Last modified December 3 22:32 EST 2021. Contains 349468 sequences. (Running on oeis4.)