∫ 0 1 k = 0 ∏ ∞ ( 1 − q 4 ( k + 1 ) ) ( 1 + q 2 k ) d q Express your solution up to 3 decimal places.
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By the Jacobi Triple Product formula, we have k = 0 ∏ ∞ ( 1 − q 4 ( k + 1 ) ) ( 1 + q 2 k ) = k = 1 ∏ ∞ ( 1 − q 4 k ) ( 1 + q 2 k − 2 ) = k = 1 ∏ ∞ ( 1 − q 2 k ) ( 1 + q 2 k − 2 ) ( 1 + q 2 k ) = m ∈ Z ∑ q m 2 + m for 0 < q < 1 , and hence ∫ 0 1 k = 0 ∏ ∞ ( 1 − q 4 ( k + 1 ) ) ( 1 + q 2 k ) d q = m ∈ Z ∑ m 2 + m + 1 1 = 3 2 π tanh ( 2 π 3 ) making the answer 3 . 5 9 6 2 9 4 5 6 1 .
@Mark Hennings Sir, the last sum was evaluated by me using the Series representation of the Digamma Function..........Is there any other method???
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Integrate z 2 + z + 1 cot π z around the square contour S n with vertices ( n + 2 1 ) ± ( n + 2 1 ) i and − ( n + 2 1 ) ± ( n + 2 1 ) i . There exists K > 0 such that ∣ cot π z ∣ ≤ K for all z ∈ S n for all n ∈ N , so the value of the sum now comes from Cauchy's Residue Theorem and taking a limit: m = − ∞ ∑ ∞ m 2 + m + 1 1 + π R e s z = ω z 2 + z + 1 cot π z + π R e s z = ω 2 z 2 + z + 1 cot π z = 0 where ω is a primitive cube root of unity.
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Although my solution is same as that of sir Mark Hennings , for the sake of remembrance of Ramanujan I'll make use of his theta function. we know that the Ramanujan theta function is defined as f ( a , b ) = n = − ∞ ∑ ∞ a 2 n ( n + 1 ) b 2 n ( n − 1 ) = ( − a ; a b ) ∞ ( − b ; a b ) ∞ ( a b ; a b ) ∞ when a = q 2 and b = 1 we have f ( q 2 , 1 ) = n = − ∞ ∑ ∞ q n 2 + n = ( − q 2 ; q 2 ) ∞ ( − 1 ; q 2 ) ∞ ( q 2 ; q 2 ) ∞ where ( a ; q ) ∞ = ∏ k = 0 ∞ ( 1 − a q k ) is a pochammer notation. we came to see that the following product equates to an infinite bilateral series.Therefore we have the result as ∫ 0 1 n = − ∞ ∑ ∞ q n 2 + n d q = n = − ∞ ∑ ∞ n 2 + n + 1 1 = i 3 − π ( cot ( π ( 2 1 + i 3 ) ) − cot ( π ( 2 1 − i 3 ) ) ) = 3 . 5 9 6 2 9 4 5 . .