APPLICATIONS 225 To begin, we note that the gamma function is the solution to the difference equation Ak+1 = kak. (7.80) This follows directly from the results of Section 2.4. To determine the dom- inant or leading behavior of the solution to equation (7.80), we rewrite it in the form ak = eSk (7.81) Substitution of equation (7.81) into (7.80) gives Sk+1 - Sk = ln k, (7.82) whose exact solution is k-1 Sk = S1 + Σhr. (7.83) r=1 Since our interest is in k → ∞, the dominant behavior of Sk can be calculated by approximating the sum by an integral; therefore, In t dt ~ k In k. (7.84) Consequently, ar takes the form ak = k*bk, (7.85) where br is an unknown function whose dominant behavior will now be de- termined. To do this, we substitute equation (7.85) into equation (7.80) and obtain -(k+1) (1+ ) bk+1 = (7.86) Using the fact that lim (1+ h)'/h = e, (7.87) it follows that -(k+1) 1 (1+) lim (7.88) and for k → ∞, we have (). bk+1 ~ (7.89) Thus, the dominant behavior of br is given by the expression br ~ e-k (7.90) If we now set br = e¬kCk; (7.91)

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Explain the determine yellow

ences (World Scientific, Singapore, 2004), Chapter 7.
7.2.4
Approximating Factorials
For integer k, the gamma function reduces to the factorial expression
T(k)
(k – 1)!.
(7.77)
Of great interest for many calculations in applied mathematics, the sciences,
and engineering is the representation of the factorial for large values of k.
This corresponds to k → , and such a representation is called an asymptotic
expansion. For equation (7.77), the asymptotic expansion is
2T
k
T(k) = (k – 1)! ~
k
A1
1+
k
A1
k2
Аз
+..
k3
(7.78)
where the symbol “n" will mean "k → oo." In this equation, the constants
A1, A2, and A3 are
1
Aj =
12'
1
A2 =
288
139
A3 =
(7.79)
51, 840
The above expansion is called the Stirling series.
We now derive equation (7.78) except for an overall multiplicative con-
stant.
APPLICATIONS
225
To begin, we note that the gamma function is the solution to the difference
equation
ak+1 = kak.
(7.80)
This follows directly from the results of Section 2.4. To determine the dom-
inant or leading behavior of the solution to equation (7.80), we rewrite it in
the form
ak = eSk
(7.81)
Substitution of equation (7.81) into (7.80) gives
Sk+1 – Sk = In k,
(7.82)
whose exact solution is
k-1
Sk = S1 +> Inr.
(7.83)
r=1
Since our interest is in k →x, the dominant behavior of Sk can be calculated
by approximating the sum by an integral; therefore,
Sk -
In t dt ~ k In k.
(7.84)
Consequently, ak takes the form
k* bk,
(7.85)
ak =
where br is an unknown function whose dominant behavior will now be de-
termined. To do this, we substitute equation (7.85) into equation (7.80) and
obtain
-(k+1)
1
bk+1 =
1+-
br.
(7.86)
(†+)--
Using the fact that
lim (1+ h)'/h = e,
(7.87)
it follows that
-(k+1)
1
1
lim
1+
(7.88)
e
and for k → ∞, we have
bk+1 ~
bk.
(7.89)
Thus, the dominant behavior of br is given by the expression
bk ~e-k.
(7.90)
If we now set
br = e-kck
(7.91)
Ck;
Transcribed Image Text:ences (World Scientific, Singapore, 2004), Chapter 7. 7.2.4 Approximating Factorials For integer k, the gamma function reduces to the factorial expression T(k) (k – 1)!. (7.77) Of great interest for many calculations in applied mathematics, the sciences, and engineering is the representation of the factorial for large values of k. This corresponds to k → , and such a representation is called an asymptotic expansion. For equation (7.77), the asymptotic expansion is 2T k T(k) = (k – 1)! ~ k A1 1+ k A1 k2 Аз +.. k3 (7.78) where the symbol “n" will mean "k → oo." In this equation, the constants A1, A2, and A3 are 1 Aj = 12' 1 A2 = 288 139 A3 = (7.79) 51, 840 The above expansion is called the Stirling series. We now derive equation (7.78) except for an overall multiplicative con- stant. APPLICATIONS 225 To begin, we note that the gamma function is the solution to the difference equation ak+1 = kak. (7.80) This follows directly from the results of Section 2.4. To determine the dom- inant or leading behavior of the solution to equation (7.80), we rewrite it in the form ak = eSk (7.81) Substitution of equation (7.81) into (7.80) gives Sk+1 – Sk = In k, (7.82) whose exact solution is k-1 Sk = S1 +> Inr. (7.83) r=1 Since our interest is in k →x, the dominant behavior of Sk can be calculated by approximating the sum by an integral; therefore, Sk - In t dt ~ k In k. (7.84) Consequently, ak takes the form k* bk, (7.85) ak = where br is an unknown function whose dominant behavior will now be de- termined. To do this, we substitute equation (7.85) into equation (7.80) and obtain -(k+1) 1 bk+1 = 1+- br. (7.86) (†+)-- Using the fact that lim (1+ h)'/h = e, (7.87) it follows that -(k+1) 1 1 lim 1+ (7.88) e and for k → ∞, we have bk+1 ~ bk. (7.89) Thus, the dominant behavior of br is given by the expression bk ~e-k. (7.90) If we now set br = e-kck (7.91) Ck;
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