Hindawi Publishing Corporation International Journal of Mathematics and Mathematical Sciences Volume 2011, Article ID 976723, 15 pages doi:10.1155/2011/976723 Research Article A Rademacher-Type Formula for podn Vyacheslav Kiria-Kaiserberg Department of Mathematical Sciences, Georgia Southern University, Statesboro, GA 30458, USA Correspondence should be addressed to Vyacheslav Kiria-Kaiserberg, vslkrg@yahoo.com Received 24 May 2011; Accepted 7 September 2011 Academic Editor: Marianna Shubov Copyright q 2011 Vyacheslav Kiria-Kaiserberg. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A Rademacher-type formula for the Fourier coefficients of the generating function for the partitions of n where no odd part is repeated is presented. 1. Partitions A partition of a positive integer n is a representation of n as a sum of positive integers where order of summands parts does not matter. Let pn represent the number of partitions of n. In 1937, Rademacher 1, 2 was able to express pn as a convergent series: ⎛ ⎞ ∞ sinh − 1/24 2/3n π/k d ⎜ 1 ⎟ Ak n k ⎝ pn √ ⎠, dn π 2 k1 n − 1/24 1.1 where Ak n eπish,k−2nh/k 0≤h<kgcdh,k1 1.2 is a Kloosterman sum and sh, k k−1 r hr r1 is a Dedekind sum. k hr 1 − − k k 2 1.3 2 International Journal of Mathematics and Mathematical Sciences In 2011, Bruinier and Ono 3 announced a new formula that expresses pn as a finite sum. 1.1. Formula for pn Let ∞ ∞ pnqn f q : 1 1 − qm m1 n0 1.4 be Euler’s generating function for pn. H. Rademacher used the classical circle method to find the coefficients of qn . There are many other infinite products to which this method could be applied. We introduce one of these infinite products here and derive the formula for the coefficients of qn . Define 2 2 ∞ 1 − qm f q G q : . 2 2m f q m1 1 − q 1.5 Let pj denote the coefficient of qj in the expansion of Gq, that is, ∞ G q p j qj . 1.6 j0 We will find a closed expression for pj. Note that ∞ ∞ 1 q2m−1 podnqn , G −q 2m 1 − q n0 m1 1.7 where podn equals the number of partitions of n where no odd part is repeated. Thus podn −1n pn −1n ∞ 2 ω2 h, k/2 −2πinh/k d e k π k1k,22 ωh, k dn 0≤h<kh,k1 ⎞ √ sinh π 8n − 1/2k ⎟ ⎜ ×⎝ √ ⎠ 8n − 1 ⎛ 1.8 International Journal of Mathematics and Mathematical Sciences 3 which is simpler than the one given by Sills 4, page 4, Equation 1.13 in 2010: ∞ ωh, kω4h/k, 4, k/k, 4 2 4, k podn k 1 − −1k π k1 4 ω2h/k, 2, k/k, 2 0≤h<kh,k1 ⎛ ⎞ sinh π 48n − 1/4k k, d ⎜ ⎟ × e−2πinh/k √ ⎝ ⎠, dn 8n − 1 1.9 where ωh, k is defined as ωh, k exp πi hr 1 . − − k k 2 k−1 r hr r1 k 1.10 2. Evaluation of the Path Integral 2.1. Convergence and Cauchy Residue Theorem Considering q as a complex variable in ∞ ∞ ∞ 1 − qm 1 1 m , 2 m 2m 2m 1−q 1 − q2m m1 1 − q m1 1 q m1 1 − −q 2.1 we see from the right-hand side that infinite product and thus also infinite series are convergent for |q| < 1 since ∞ n qk 1 , 1 − qk n0 2.2 is a geometric series which converges for |q| < 1 for any fixed k ≥ 1. Next, we note that from ∞ p j qj , G q 2.3 j0 we get that ∞ p G q j qj n1 qn1 j0 q if 0 < q < 1. 2.4 4 International Journal of Mathematics and Mathematical Sciences The series on the right side of 2.4 is a Laurent series of Gq/qn1 . It has a pole of order n 1 at q 0 with residue pn. Applying Cauchy’s Residue Theorem we get that 1 pn 2πi C G q 1 dq 2πi C qn1 2 2 f q dq, f q qn1 2.5 where C is any positively oriented simple closed countour lying inside the unit circle. 2.2. Change of the Variable The change of the variable q e2πiτ maps the unit disk |q| < 1 into an infinite vertical strip of width 1 in the τ-plane. To see this we note that from q e2πiτ we get log q 2πiτ, so τ log q/2πi. Choosing the branch cut to be 0, 1, we get logq Arg q . τ 2πi 2π 2.6 As q traverses a circle centered at q 0 of radius e−2π in the positive direction, the point τ varies from i to i 1 along a horizontal segment as could be easly deduced from 2.6. Replacing the segment by the Rademacher path composed of upper arcs of the Ford circles formed by the Farey series FN , 2.5 becomes 1 pn 2πi i1 4πiτ 2 2πie2πiτ f e i fe2πiτ e2πiτn1 dτ, 2.7 which simplifies to i1 4πiτ 2 f e pn e−2πiτn dτ, fe2πiτ i 4πiτ 2 f e e−2πiτn dτ. 2πiτ P N fe 2.8 The above can be written as P N 4πiτ 2 4πiτ 2 N f e f e −2πiτn e e−2πiτn dτ, dτ fe2πiτ fe2πiτ k1 0≤h<kh,k1 γh,k where γh, k is the upper arc of the Ford circle Ch, k. 2.9 International Journal of Mathematics and Mathematical Sciences 5 2.3. Another Change of the Variable Consider another change of variable τ h iz , k k 2.10 so that h z −ik τ − k 2.11 dz −ik dτ. 2.12 Under this transformation the Ford circle Ch, k in the τ-plane with center at h/k i1/2k2 and radius 1/2k2 is mapped to a negatively oriented circle Ck in the z-plane with center at 1/2k and radius 1/2k. This follows from the fact that any point on the Ford circle Ch, k is given by τ 1 h i 2 k 2k 1 iθ e , 2k2 0 ≤ θ < 2π. 2.13 Substitution of 2.13 into 2.11 gives z 1 iθ 1 −ie , 2k 2k 2.14 which is a circle centered at 1/2k with radius 1/2k. Now we make change of variable in 2.9. This gives pn i N k−1 k1 e−2πinh/k 0≤h<k h,k1 th,k 4πih/k−4πiz/k 2 f e 2πih/k−2πz/k e2πnz/k dz, f e sh,k 2.15 where sh,k th,k k2 kp k i, 2 2 kp k kp2 k ks − i 2 2 2 k ks k ks2 are initial and terminal points, respectively. 2.16 6 International Journal of Mathematics and Mathematical Sciences 2.4. Modular Transformation Next, we note that eπiτ/12 , f q f e2πiτ ητ 2.17 where ητ is the Dedekind eta function. Rewriting modular functional equation 5, page 96 for ητ in terms of fq fe2πiτ fe2πih/k−2πz/k we get f e 2πih/k−2πz/k −1 √ π z −z iz−1 H ωh, k exp , zf exp 2πi 12k k 2.18 with hH ≡ −1modk, h, k 1. To evaluate 2.15 we would like to express f e4πih/k−4πiz/k 2 2πiτ 2πih/k−2πz/k G e G q G e , f e2πih/k−2πz/k 2.19 in the same way we did for fq above. Two cases have to be considered: k, 2 1 and k, 2 2. When k, 2 1 we will replace h by 2h and z by 2z, and when k, 2 2, k will be replaced by k/2 in order to obtain fq2 from fq. Hence, we have ⎧ −1 −1 ⎪ ω2 2h, keπ2z −2z/6k 2zf 2 e2πii2z H2 /k ⎪ ⎪ ⎪ ⎪ , √ ⎨ ⎪ ωh, keπz−1 −z/12k zf e2πiiz−1 H2 /k 2πih/k−2πz/k G e ⎪ 2 πz−1 −z/3k 2 4πiiz−1 H1 /k ⎪ e zf ω k/2e h, ⎪ ⎪ ⎪ ⎪ , √ ⎩ ωh, keπz−1 −z/12k zf e2πiiz−1 H1 /k if k, 2 1, if k, 2 2, 2.20 which simplifies to ⎧ 2 2πii2z−1 H2 /k ⎪ e f 2 ⎪ ⎪ ω 2h, k −πz/4k √ e z 2πiiz−1 H /k , ⎨2 ⎪ 2 ωh, k 2πih/k−2πz/k f e G e ⎪ ⎪ ω2 h, k/2 √ ⎪ −1 −1 ⎪ ⎩ eπz −z/4k zG e2πiiz H1 /k , ωh, k if k, 2 1, 2.21 if k, 2 2, where hHj ≡ −1modk and j | Hj for j 1, 2. We return to evaluation of 2.15. To proceed we note that −1 −1 G e2πiiz H1 /k 1 G e2πiiz H1 /k − 1 . 2.22 International Journal of Mathematics and Mathematical Sciences 7 Rewriting 2.15 in terms of 2.21 and 2.22 we obtain N pn i k−1 k1k,21 N i 2 0≤h<kh,k1 k−1 k1k,22 ω 2h, k −2πinh/k e ωh, k 2 2 2πii2z−1 H2 /k √ f e z 2πiiz−1 H /k eπz/k2n−1/4 dz 2 f e sh,k th,k ω2 h, k/2 −2πinh/k e ωh, k 0≤h<kh,k1 th,k √ −1 z 1 G e2πiiz H1 /k − 1 sh,k × eπz/k2n−1/4π/4zk dz N 2i k−1 k1k,21 N i ω 2h, k −2πinh/k e ωh, k 0≤h<kh,k1 k−1 k1k,22 2 2 2πii2z−1 H2 /k e f √ z 2πiiz−1 H /k eπz/k2n−1/4 dz 2 f e sh,k th,k ω2 h, k/2 −2πinh/k e J1 h, k J2 h, k, ωh, k 0≤h<kh,k1 2.23 where J1 h, k J2 h, k th,k √ sh,k th,k zeπz/k2n−1/4π/4zk dz , √ 2πiiz−1 H1 /k − 1 eπz/k2n−1/4π/4zk dz. z G e 2.24 sh,k 2.5. Estimation of the First Term We will estimate the first term in 2.23 and will show that it is small for large N. To do this we change variable again by letting ξ zk. Then the first term in 2.23 becomes 2i N k−5/2 k1k,21 ω 2h, k −2πinh/k e ωh, k 0≤h<kh,k1 2 t∗ h,k s∗h,k −1 f 2 e2πii2ξ/k H2 /k 2 ξ eπξ/k 2n−1/4 dξ, −1 2πiiξ/k H /k 2 f e 2.25 where s∗h,k kkp k2 i, 2 2 2 k kp k kp2 2.26 t∗h,k k2 kks − i, 2 2 2 k ks k ks2 2.27 are initial and terminal points obtained from 2.16, respectively. Under this change of variable circle Ck in z-plane with center at 1/2k and radius 1/2k is mapped to a circle Ck∗ 8 International Journal of Mathematics and Mathematical Sciences in ξ-plane centered at 1/2 with radius 1/2. Note also that the mapping w 1/ξ maps the circle Ck∗ and its interior onto a half-plane Rw ≥ 1 where Rw denotes the real part of complex variable w and Iw is the imaginary part. From elementary complex analysis we have that Rw x/x2 y2 and Iw −y/x2 y2 , where x iy ξ. It is readily seen that the segment 0 < x ≤ 1 in the ξ-plane is mapped to an infinite strip 1, ∞ in the w-plane. So, it follows that inside and on the circle Ck∗ we have that 0 < Rξ ≤ 1 and R1/ξ ≥ 1. We now show that R1/ξ 1 on the circle Ck∗ . To see this note that in the polar form ξ 1/2 1/2eiθ on Ck∗ , 0 ≤ θ ≤ 2π. From this we get that 1 2 2 ξ 1 eiθ 1 cos θ i sin θ 21 cos θ − i sin θ 1 cos θ2 sin2 θ 21 cos θ 2 sin θ −i 2 2 cos θ 2 2 cos θ 1−i 2.28 sin θ . 1 cos θ So, R1/ξ 1. Furthermore, we may move path of integration from the arc joining s∗h,k and t∗h,k to a segment connecting these two points on the circle Ck∗ . By 5, page 104, Theorem 5.9 the √ length of the path of integration is bounded by 2 2k/N, and on the segment connecting s∗h,k √ and t∗h,k , |ξ| < 2k/N. Next, let us define p∗ m by −1 f 2 e2πii2ξ/k H2 /k p∗ mqm , −1 f e2πiiξ/k H2 /k m0 ∞ 2.29 which is a part of the integrand in 2.25. Then, estimating the integrand in 2.25 we get 2 2πii2ξ/k−1 H2 /k f e ξeπξ/k2 2n−1/4 × −1 −1 2πiiξ/k H /k 2 f e ⎛ ⎞ −1 ∞ H 2πim iξ/k 2 2 ⎟ ⎜ |ξ|1/2 eπξ/k 2n−1/4 × p∗ m exp⎝ ⎠ m1 k ∞ 2πm 2πimH2 1/2 πξ/k2 2n−1/4 ∗ |ξ| e exp × p m exp − m1 ξ k International Journal of Mathematics and Mathematical Sciences ∞ 2πm 2πimH 2 2 ≤ |ξ|1/2 eπ/k 2n−1/4Rξ p∗ m exp − exp m1 ξ k ≤ |ξ| 1/2 2πn e ≤ |ξ|1/2 e2πn 9 ∞ p∗ m exp −2πmR 1 ξ m1 ∞ p∗ me−2πm m1 |ξ|1/2 e2πn ∞ p∗ mym , where y e−2π m1 c|ξ|1/2 , 2.30 where c e2πn ∞ p∗ mym . 2.31 m1 Note that c does not depend on ξ or N. It depends on n, but n remains fixed in the above analysis. So, ∗ t h,k s∗ h,k −1 √ 1/2 √ f 2 e2πii2ξ/k H2 /k 2k 2 2N 1/2 πξ/k2 2n−1/4 ξ dξ ≤ c|ξ| ≤ c e −1 N N 2πiiξ/k H /k 2 f e 2.32 < αk3/2 N −3/2 , for some constant α, and we have that ∗ N ω2 2h, k −2πinh/k t h,k −5/2 2i e k ωh, k k1k,21 s∗ h,k 0≤h<kh,k1 −1 f 2 e2πii2ξ/k H2 /k πξ/k2 2n−1/4 ξ e dξ −1 2πiiξ/k H /k 2 f e ≤2 N 2.33 αk−1 N −3/2 k1k,210≤h<kh,k1 ≤ 2αN −3/2 N 1 2αN −1/2 . k1k,21 This completes the estimation of the first term in 2.23. We proceed to the second term. 10 International Journal of Mathematics and Mathematical Sciences 2.6. Estimation of the Second Term First, we will show that J2 h, k th,k √ 2πiiz−1 H1 /k − 1 eπz/k2n−1/4π/4zk dz z G e 2.34 sh,k is small for large N. Making change of variable ξ zk as before, we get that J2 h, k k −3/2 t∗ h,k s∗h,k −1 2 ξ G e2πiiξ/k H1 /k − 1 eπξ/k 2n−1/4π/4ξ dξ, 2.35 where s∗h,k and t∗h,k are as in 2.26, respectively. As before, we define p∗∗ m by ∞ −1 p∗∗ mqm G e2πiiξ/k H1 /k − 1. m0 Then, estimating the integrand, we see that ξeπξ/k2 2n−1/4π/4ξ × G e2πiiξ/k−1 H1 /k − 1 ∞ 2πimik/ξ H1 1/2 πξ/k2 2n−1/4 π/4ξ ∗∗ |ξ| e e − 1 × p m exp m0 k ∞ −2πm 2πimH1 1/2 π/k2 2n−1/4Rξ π/4R1/ξ ∗∗ ≤ |ξ| e e exp p m exp m1 ξ k ≤ |ξ|1/2 e2πn eπ/4R1/ξ ∞ p∗∗ m exp −2πmR 1 ξ m1 |ξ|1/2 e2πn ∞ p∗∗ m exp −2πm π R 1 4 ξ m1 ≤ |ξ|1/2 e2πn ∞ p∗∗ m exp −2πm π 4 m1 |ξ|1/2 e2πn ∞ p∗∗ me−π/48m−1 m1 ≤ |ξ|1/2 e2πn ∞ p∗∗ 8m − 1e−π/48m−1 m1 2.36 International Journal of Mathematics and Mathematical Sciences |ξ|1/2 e2πn ∞ p∗∗ 8m − 1x8m−1 , 11 where x e−π/4 m1 b|ξ|1/2 , 2.37 where ∞ p∗∗ 8m − 1x8m−1 . b e2πn 2.38 m1 Note that b does not depend on ξ or N. It depends on n, but n is fixed. It follows, therefore, that √ 2k N |J2 h, k| ≤ b 1/2 √ 2 2N < βk3/2 N −3/2 , N 2.39 for some constant β. Then we have that 2πinh N N 2 − ω h, k/2 −5/2 k J2 h, k < e k βk−1 N −3/2 i k1k,22 ωh, k 0≤h<kh,k1 k1k,220≤h<kh,k1 N ≤ βN −3/2 2.40 1 βN −1/2 . k1k,22 Combining the results from 2.33 and 2.40 we have that N pn i k−5/2 k1k,22 N i k k1k,22 −5/2 ω2 h, k/2 −2πinh/k e J1 h, k O βN −1/2 2αN −1/2 ωh, k 0≤h<kh,k1 ω2 h, k/2 −2πinh/k e J1 h, k O N −1/2 . ωh, k 0≤h<kh,k1 2.41 Finally, we turn our attention to J1 h, k k−3/2 t∗ h,k ξeπξ/k s∗h,k 2 2n−1/4π/4ξ dξ. 2.42 We note that J1 h, k Ck∗ − 0 s∗h,k − t∗ h,k 0 Ck∗ −S1 − S2 , 2.43 12 International Journal of Mathematics and Mathematical Sciences where Ck∗ is a circle in the ξ-plane centered at 1/2 with radius 1/2, as before. It is easily seen that the length of the arc connecting 0 and s∗h,k is less then 2π ∗ sh,k 2 √ k ≤ π s∗h,k ≤ π 2 . N 2.44 From the discussion above we know that R1/ξ 1 and 0 < Rξ ≤ 1 on Ck∗ . So, the integrand in S1 could be estimated as ξeπξ/k2 2n−1/4π/4ξ |ξ|1/2 eπξ/k2 2n−1/4 eπ/4ξ |ξ|1/2 eπ/k ≤ 21/4 2 2n−1/4Rξ π/4R1/ξ e 2.45 k1/2 2πn π/4 e e . N 1/2 2.7. Combining the Results We combine the results in 2.44 and 2.45 to get |S1 | < γk3/2 N −3/2 , 2.46 where γ is a constant. We can obtain similar estimate for S2 and, as before, we get an error term ON −1/2 in the formula for pn. Therefore, we can write N pn i k−5/2 k1k,22 × Ck∗ ξe ω2 h, k/2 −2πinh/k e ωh, k 0≤h<kh,k1 πξ/k2 2n−1/4π/4ξ dξ O N −1/2 2.47 . Letting N → ∞ we have that ∞ pn i k−5/2 k1k,22 × Ck∗ ξe ω2 h, k/2 −2πinh/k e ωh, k 0≤h<kh,k1 πξ/k2 2n−1/4π/4ξ 2.48 dξ. We introduce another change of variable ξ 1 , w dξ − 1 . w2 2.49 International Journal of Mathematics and Mathematical Sciences 13 Then 2.48 becomes ∞ 1 ω2 h, k/2 −2πinh/k e k−5/2 i k1k,22 ωh, k 0≤h<kh,k1 pn × 1∞i w−5/2 eπ/k 2 2n−1/41/wπw/4 2.50 dw. 1−∞i Let t πw/4 in 2.50, then the above becomes pn 2π × π 3/2 8 ∞ k−5/2 k1k,22 π/4∞i t−5/2 etπ 2 ω2 h, k/2 −2πinh/k 1 e ωh, k 2πi 0≤h<kh,k1 /4k 2n−1/41/t 2 2.51 dt. π/4−∞i 2.8. Bessel Function In Watson’s Treatise on Bessel functions 6, page 181, we find a formula equivalent to the following: c∞i 1/2zν 2πi Iν z t−ν−1 etz /4t dt, 2 if c > 0, Rν > 0. 2.52 c−∞i Let z 2 ! "1/2 π2 1 2n − 4 4k2 2.53 and ν 3/2. Then we have pn 2π π 3/2 8 ∞ k−5/2 k1k,22 ω2 h, k/2 −2πinh/k e ωh, k 0≤h<kh,k1 ⎛ π π −3/2 2n − 1/4−3/4 × I3/2 ⎝ 2n − −3/4 −3/2 k 4 k ⎞ 1⎠ 4 ⎛ ∞ 2 2π2n − 1/4−3/4 π ω k/2 h, e−2πinh/k I3/2 ⎝ 2n − k−1 √ ωh, k k 8 k1k,22 0≤h<kh,k1 ⎞ 1⎠ . 4 2.54 14 International Journal of Mathematics and Mathematical Sciences Note that Bessel functions of this order can be expressed as I3/2 z 2z d sinh z . π dz z 2.55 2z cosh z sinh z − . π z z2 2.56 Expanding 2.55 we have that I3/2 z Substituting 2.53 into 2.56, we get √ π π 8n − 1 1 1/2 I3/2 2n − I3/2 z I3/2 k 4 2k # ⎛ ⎞ √ √ $ √ $ 2 π 8n − 1/2k 8n − 1/2k 8n − 1/2k cosh π sinh π % ⎜ ⎟ − √ ⎝ √ 2 ⎠ π π 8n − 1/2k π 8n − 1/2k ⎛ √ ⎞ √ 8n − 1/2k 2 cosh π 8n − 1/2k 4k/π sinh π 8n − 1 ⎜ ⎟ − √ ⎝ ⎠ π√ π k 8n − 1 8n − 1 k k ⎛ ⎞ √ √ 8n − 1/2k 4k sinh π π 8n − 1 1 ⎟ ⎜ − √ ⎠. ⎝2 cosh √ 2k π 8n − 1 π 8n − 1/k 2.57 1/4 Multiplying 2.57 by 2π2n − 1/4−3/4 2π , √ 8 8n − 13/4 2.58 we get √ √ √ 2 2 cosh π 8n − 1/2k − 4k sinh π 8n − 1/2k /π 8n − 1 8n − 13/4 √ 8n − 1/k . 2.59 International Journal of Mathematics and Mathematical Sciences 15 2.9. Final Form Finally, we rewrite 2.54 in terms of 2.59 to get ∞ pn k−1 k1k,22 × ω2 h, k/2 −2πinh/k e ωh, k 0≤h<kh,k1 √ √ √ 2 2 cosh π 8n − 1/2k − 4k sinh π 8n − 1/2k /π 8n − 1 8n − 13/4 √ 8n − 1/k 2.60 . Thus, podn −1n ∞ k−1 k1k,22 × ω2 h, k/2 −2πinh/k e ωh, k 0≤h<kh,k1 √ √ √ 2 2 cosh π 8n − 1/2k − 4k sinh π 8n − 1/2k /π 8n − 1 8n − 13/4 √ 8n − 1/k 2.61 , or equivalently podn −1n ∞ 2 π k1k,22 ⎛ ⎞ √ 8n − 1/2k sinh π ω h, k/2 −2πinh/k d ⎜ ⎟ e k √ ⎝ ⎠. ωh, k dn 8n − 1 0≤h<kh,k1 2 2.62 Acknowledgment The authors wishes to express gratitude and appreciation to Dr. A. Sills for his suggestions and criticism. References 1 H. Rademacher, “On the expansion of the partition function in a series,” Annals of Mathematics, vol. 44, no. 2, pp. 416–422, 1943. 2 H. Rademacher, “On the partition function pn,” Proceedings of the London Mathematical Society, vol. 43, no. 2, pp. 241–254, 1937. 3 J. Bruinier and K. Ono, “An algebraic formula for the partition function,” http://www.aimath.org/ news/partition/brunier-ono. 4 A. V. Sills, “A Rademacher type formula for partitions and overpartitions,” International Journal of Mathematics and Mathematical Sciences, vol. 2010, Article ID 630458, 21 pages, 2010. 5 T. M. Apostol, Modular Functions and Dirichlet Series in Number Theory, vol. 41 of Graduate Texts in Mathematics, Springer, New York, NY, USA, 2nd edition, 1990. 6 G. N. Watson, A Treatise on the Theory of Bessel Functions, Cambridge University Press, Cambridge, UK, 2nd edition, 1944. 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