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A198200
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G.f.: q-cosh(x,q)^2 - q-sinh(x,q)^2 at q=-x.
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6
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1, 0, 1, 2, 3, 6, 10, 16, 28, 48, 79, 130, 215, 356, 587, 960, 1566, 2558, 4176, 6804, 11066, 17978, 29198, 47406, 76916, 124716, 202152, 327600, 530775, 859734, 1392265, 2254336, 3649840, 5908632, 9564377, 15480706, 25055322, 40549980, 65624224, 106199306, 171856555, 278099872
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OFFSET
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0,4
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COMMENTS
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This sequence illustrates in part the identities:
* q-cosh(x,q)^2 - q-sinh(x,q)^2 = e_q(x,q) / E_q(x,q),
* q-Cosh(x,q)^2 - q-Sinh(x,q)^2 = E_q(x,q) / e_q(x,q).
Here the following q-analogs are employed (see MathWorld links):
q-cosh(x,q) = Sum_{n>=0} x^(2*n)/faq(2*n,q),
q-sinh(x,q) = Sum_{n>=0} x^(2*n+1)/faq(2*n+1,q),
and the dual expressions:
q-Cosh(x,q) = Sum_{n>=0} q^(n*(2*n-1))*x^(2*n)/faq(2*n,q),
q-Sinh(x,q) = Sum_{n>=0} q^(n*(2*n+1))*x^(2*n+1)/faq(2*n+1,q),
along with the dual q-exponential functions of x:
e_q(x,q) = Sum_{n>=0} x^n/faq(n,q),
E_q(x,q) = Sum_{n>=0} q^(n*(n-1)/2) * x^n/faq(n,q),
where
faq(n,q) = Product_{k=1..n} (q^k-1)/(q-1) is the q-factorial of n.
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LINKS
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FORMULA
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(1) G.f.: e_q(x,q) / E_q(x,q) at q=-x, where
e_q(x,-x) = Sum_{n>=0} x^n/Product_{k=1..n} (1-(-x)^k)/(1+x)),
E_q(x,-x) = Sum_{n>=0} (-x)^(n*(n-1)/2) * x^n/Product_{k=1..n} (1-(-x)^k)/(1+x)).
(2) G.f.: exp( Sum_{n>=1} (1+x)^(2*n)/(1-x^(2*n)) * x^(2*n)/n ).
(3) G.f.: Product_{n>=1} 1/(1 - x^(2*n)*(1+x)^2).
(4) Limit a(n+1)/a(n) = phi = (sqrt(5)+1)/2 with Limit a(n)/phi^n = 0.75149846280232258786564518960536101986114488526276981847216113150440...
Limit a(n)/phi^n = phi / (2*sqrt(5)) * Product_{k>=2} 1/(1 - phi^(2 - 2*k)). - Vaclav Kotesovec, Oct 22 2020
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EXAMPLE
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G.f.: A(x) = 1 + x^2 + 2*x^3 + 3*x^4 + 6*x^5 + 10*x^6 + 16*x^7 + 28*x^8 +...
The g.f. may be expressed by:
(0) A(x) = q-cosh(x,q)^2 - q-sinh(x,q)^2 at q=-x, where
q-cosh(x,-x) = 1 + x^2 + x^3 + 2*x^4 + 4*x^5 + 6*x^6 + 9*x^7 + 15*x^8 + 25*x^9 + 41*x^10 + 66*x^11 + 105*x^12 +...
q-sinh(x,-x) = x + x^3 + 2*x^4 + 3*x^5 + 5*x^6 + 8*x^7 + 13*x^8 + 23*x^9 + 39*x^10 + 62*x^11 + 99*x^12 +...
q-cosh(x,-x)^2 = 1 + 2*x^2 + 2*x^3 + 5*x^4 + 10*x^5 + 17*x^6 + 30*x^7 + 54*x^8 + 96*x^9 + 170*x^10 + 296*x^11 + 510*x^12 +...
q-sinh(x,-x)^2 = x^2 + 2*x^4 + 4*x^5 + 7*x^6 + 14*x^7 + 26*x^8 + 48*x^9 + 91*x^10 + 166*x^11 + 295*x^12 +...
(1) A(x) = e_q(x,q) / E_q(x,q) at q=-x, where
e_q(x,-x) = 1 + x + x^2 + 2*x^3 + 4*x^4 + 7*x^5 + 11*x^6 + 17*x^7 + 28*x^8 + 48*x^9 + 80*x^10 + 128*x^11 + 204*x^12 +...
E_q(x,-x) = 1 + x - x^3 - x^4 - x^5 - 2*x^6 - 3*x^7 - 3*x^8 - 2*x^9 + 2*x^11 + 2*x^12 +...
(2) log(A(x)) = (1+x)^2/(1-x^2)*x^2 + (1+x)^4/(1-x^4)*x^4/2 + (1+x)^6/(1-x^6)*x^6/3 + (1+x)^8/(1-x^8)*x^8/4 +...
(3) A(x) = 1/((1 - x^2*(1+x)^2) * (1 - x^4*(1+x)^2) * (1 - x^6*(1+x)^2) * (1 - x^8*(1+x)^2) * (1 - x^10*(1+x)^2) *...).
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MATHEMATICA
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nmax = 50; CoefficientList[Series[Product[1/(1 - x^(2*k)*(1+x)^2), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Oct 22 2020 *)
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PROG
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(PARI) /* (0) G.f. q-cosh(x, q)^2 - q-sinh(x, q)^2 at q=-x: */
{a(n)=local(cosh_q=sum(k=0, n, x^(2*k)/(prod(j=1, 2*k, (1-(-x)^j)/(1+x))+x*O(x^n))), sinh_q=sum(k=0, n, x^(2*k+1)/(prod(j=1, 2*k+1, (1-(-x)^j)/(1+x))+x*O(x^n)))); polcoeff(cosh_q^2-sinh_q^2, n)}
(PARI) /* (1) G.f. e_q(x, q) / E_q(x, q) at q=-x: */
{a(n)=local(e_q=sum(k=0, n, x^k/(prod(j=1, k, (1-(-x)^j)/(1+x))+x*O(x^n))), E_q=sum(k=0, n, (-x)^(k*(k-1)/2)*x^k/(prod(j=1, k, (1-(-x)^j)/(1+x))+x*O(x^n)))); polcoeff(e_q/E_q, n)}
(PARI) /* (1) G.f. e_q(x, q) / E_q(x, q) at q=-x: */
{a(n)=local(e_q=exp(sum(k=1, n, x^k*(1+x)^k/(1-(-x)^k)/k)+x*O(x^n)), E_q=exp(sum(k=1, n, -(-x)^k*(1+x)^k/(1-(-x)^k)/k)+x*O(x^n))); polcoeff(e_q/E_q, n)}
(PARI) /* (2) G.f. exp( Sum_{n>=1} (1+x)^(2*n)/(1-x^(2*n)) * x^(2*n)/n): */
{a(n)=polcoeff( exp( sum(m=1, n\2+1, (1+x)^(2*m)/(1-x^(2*m)+x*O(x^n))*x^(2*m)/m)), n)}
(PARI) /* (3) G.f. Product_{n>=1} 1/(1 - x^(2*n)*(1+x)^2): */
{a(n)=polcoeff(1/prod(k=1, n, 1-(1+x)^2*x^(2*k)+x*O(x^n)), n)}
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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