Atlas › Modular forms › Weight 2 › Level 81 › 81.2.a

Newspace 81.2.a: $S_{2}^{\mathrm{new}}(\Gamma_0(81))$

Newforms

Label	Dim	Coefficient field	Trace form	AL signs	Sign	CM	Elliptic curve
81.2.a.a	2	$\mathbb{Q}(\sqrt{3})$	$2q + 2q^{4} + 4q^{7} + O(q^{8})$

Hecke characteristic polynomials

The eigenvalue of $T = T_{2} + 3T_{5}$ generates each orbit's coefficient field; on the newspace its characteristic polynomial factors over $\mathbb{Q}$ as one irreducible factor per orbit:

Orbit	Characteristic polynomial of $T$
81.2.a.a	$x^{2} - 12$

Dimensions

Newforms $S_{2}^{\mathrm{new}}(\Gamma_0(81))$	2
Old forms	2
Cusp forms $S_{2}(\Gamma_0(81))$	4
Eisenstein series $E_{2}(\Gamma_0(81))$	11
Modular forms $M_{2}(\Gamma_0(81))$	15

Decomposition of the old subspace

$S_{2}^{\mathrm{old}}(\Gamma_0(81)) =$ $S_{2}^{\mathrm{new}}(\Gamma_0(27))^{\oplus 2}$

Provenance

Stored in the atlas: computed by Sagebrush's modular symbols engine in 0.107 s (one thread) and proven: the dimensions are certified, and the characteristic polynomial of $T$ over $\mathbb{Z}$ is reconstructed by CRT past a rigorous coefficient bound.

The engine's checks

dim M^+ at levels [1, 3, 9, 27, 81]: [0, 1, 2, 4, 10] (certified at ell = 2147483179)
T = [(2, 1), (5, 3)]; h of degree 2 = 1 x 2 (phi(ord eps) x dim S_k^new); 1 primes, 0 rejected

Open in the notebook · JSON · stored data

Recompute it yourself

In Sage mode at sagebrush.space (or npx sagebrush --sage):

newform_orbits(81, 2)

With the Rust engine (source), in engine/:

cargo run --release -p sagebrush-web --example atlas -- 81:2 1000

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