A369673 a(n) = Product_{k=0..n} (2^k + 2^(n-k)).

2, 9, 100, 2916, 231200, 50808384, 31258240000, 54112148361216, 264265663201280000, 3645603832850650497024, 142153785549232537600000000, 15673043740102659990892604030976, 4886752115388739132874502963200000000, 4309225323078788454199311474023086952546304, 10747393363422494556085100202291563069440000000000

Offset

0,1

Comments

Conjectures:

(C.1) a(n) is a square iff n is not divisible by 4.

(C.2) a(2*n+1) is not divisible by 5 for n >= 0.

(C.3) exponent of highest power of 5 dividing a(4*n) = 2*A127428(n).

(C.4) exponent of highest power of 5 dividing a(4*n+2) = 2*A127428(n+1).

From Vaclav Kotesovec, Feb 07 2024: (Start)

For q > 1, Product_{k=0..n} (q^k + q^(n-k)) ~ c * q^(3*n^2/4 + n), where

c = QPochhammer(-1, 1/q^2)^2/2 if n is even and

c = q^(1/4) * QPochhammer(-q, 1/q^2)^2 / (q + 1)^2 if n is odd.

c_even / c_odd = EllipticTheta[2, 0, 1/q] / EllipticTheta[3, 0, 1/q] = JacobiTheta2(0, 1/q) / JacobiTheta3(0, 1/q). (End)

Links

Table of n, a(n) for n=0..14.

Formula

a(n) = Product_{k=0..n} (2^k + 2^(n-k)).

a(n) = 2^(n*(n+1)) * Product_{k=0..n} (1/2^k + 1/2^(n-k)).

a(n) = 2^(n*(n+1)/2)*QPochhammer(-2^n, 1/4, 1 + n). - Stefano Spezia, Feb 06 2024

From Vaclav Kotesovec, Feb 07 2024: (Start)

a(n) ~ c * 2^(3*n^2/4 + n), where

c = 3.676982087353134... = QPochhammer(-1, 1/4)^2/2 if n is even and

c = 3.676991719144565... = 2^(1/4) * QPochhammer(-2, 1/4)^2 / 9 if n is odd.

c_even / c_odd = EllipticTheta[2, 0, 1/2] / EllipticTheta[3, 0, 1/2] = JacobiTheta2(0, 1/2) / JacobiTheta3(0, 1/2) = 0.9999973805240351337720926619... (End)

Example

a(0) = (1 + 1) = 2;
a(1) = (1 + 2)*(2 + 1) = 9;
a(2) = (1 + 2^2)*(2 + 2)*(2^2 + 1) = 100;
a(3) = (1 + 2^3)*(2 + 2^2)*(2^2 + 2)*(2^3 + 1) = 2916;
a(4) = (1 + 2^4)*(2 + 2^3)*(2^2 + 2^2)*(2^3 + 2)*(2^4 + 1) = 231200;
a(5) = (1 + 2^5)*(2 + 2^4)*(2^2 + 2^3)*(2^3 + 2^2)*(2^4 + 2)*(2^5 + 1) = 50808384;
a(6) = (1 + 2^6)*(2 + 2^5)*(2^2 + 2^4)*(2^3 + 2^3)*(2^4 + 2^2)*(2^5 + 2)*(2^6 + 1) = 31258240000;
...
RELATED SERIES.
Let F(x) be the g.f. of A369557, then
F(1/2) = 2 + 9/2^2 + 100/2^6 + 2916/2^12 + 231200/2^20 + 50808384/2^30 + 31258240000/2^42 + 54112148361216/2^56 + ... + a(n)/2^(n*(n+1)) + ... = 6.800139835051923542641455169580774467247971025...

Prog

(PARI) {a(n) = prod(k=0, n, 2^k + 2^(n-k))}
for(n=0, 15, print1(a(n), ", "))

Crossrefs

Cf. A369674, A369675, A369676, A369557, A127428.

Sequence in context: A368725 A277180 A013520 * A041239 A098610 A226391

Adjacent sequences: A369670 A369671 A369672 * A369674 A369675 A369676

Keyword

nonn

Author

Paul D. Hanna, Feb 06 2024

Status

approved