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 A000113 Number of transformation groups of order n. 1
 1, 3, 4, 3, 6, 12, 8, 6, 4, 18, 12, 12, 14, 24, 24, 6, 18, 12, 20, 18, 32, 36, 24, 24, 30, 42, 12, 24, 30, 72, 32, 12, 48, 54, 48, 12, 38, 60, 56, 36, 42, 96, 44, 36, 24, 72, 48, 24, 56, 90, 72, 42, 54, 36, 72, 48, 80, 90, 60, 72, 62, 96, 32, 12, 84, 144, 68 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A strong divisibility sequence, that is, gcd(a(n), a(m)) = a(gcd(n, m)) for all positive integers n and m. - Michael Somos, Jan 03 2017 REFERENCES B. Schoeneberg, Elliptic Modular Functions, Springer-Verlag, NY, 1974, p. 139. LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 FORMULA Let psi(m) = A001615(m) (Dedekind's psi function). Write n = 2^i*3^j*k, where (6,k) = 1 and let i' = floor(i/2) for i < 6, i' = 3 for i >= 6; let j' = 0 for j = 0 or 1, j' = 1 for j >= 2. Then a(n) = psi(n/(2^i'*3^j')) = psi(n)/(2^i'*3^j'). Multiplicative with a(2^e)=3*2^Floor[(e-1)/2] for 0=7, a(3^e)=4 for 0=3 and a(p^e)=(p+1)*p^(e-1) for p>3. - T. D. Noe, Nov 14 2006 EXAMPLE G.f. = x + 3*x^2 + 4*x^3 + 3*x^4 + 6*x^5 + 12*x^6 + 8*x^7 + 6*x^8 + 4*x^9 + ... MATHEMATICA psi[n_] := n*DivisorSum[n, MoebiusMu[#]^2/#&]; a[n_] := (i=IntegerExponent[ n, 2]; j=IntegerExponent[n, 3]; ip = If[i<6, Floor[i/2], 3]; jp = If[j<2, 0, 1]; psi[n]/(2^ip*3^jp)); Array[a, 60] (* Jean-François Alcover, Feb 04 2016 *) a[ n_] := If[ n < 1, 0, n Sum[ MoebiusMu[d]^2/d, {d, Divisors @ n}] / (2^Min[3, Quotient[IntegerExponent[n, 2], 2]] 3^Boole[1 < IntegerExponent[n, 3]]) ]; (* Michael Somos, Jan 03 2017 *) PROG (PARI) {a(n) = if( n<1, 0, n * sumdiv(n, d, moebius(d)^2 / d) / (2^min(3, valuation(n, 2)\2) * 3^(1 < valuation(n, 3))))}; /* Michael Somos, Jan 03 2017 */ CROSSREFS Sequence in context: A183100 A046897 A109506 * A069915 A033634 A111970 Adjacent sequences:  A000110 A000111 A000112 * A000114 A000115 A000116 KEYWORD nonn,easy,mult AUTHOR STATUS approved

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Last modified November 17 19:40 EST 2017. Contains 294834 sequences.