Atlas › Modular forms › Weight 2 › Level 21 › 21.2.a

Newspace 21.2.a: $S_{2}^{\mathrm{new}}(\Gamma_0(21))$

Newforms

Label	Dim	Coefficient field	Trace form	AL signs	Sign	CM	Elliptic curve
21.2.a.a	1	$\mathbb{Q}$	$q - q^{2} + q^{3} - q^{4} - 2q^{5} - q^{6} - q^{7} + O(q^{8})$	− +	$+1$		21a1 rank 0

Hecke characteristic polynomials

The eigenvalue of $T = T_{2} + T_{5}$ 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 $T$
21.2.a.a	$x + 3$

Dimensions

Newforms $S_{2}^{\mathrm{new}}(\Gamma_0(21))$	1
Old forms	0
Cusp forms $S_{2}(\Gamma_0(21))$	1
Eisenstein series $E_{2}(\Gamma_0(21))$	3
Modular forms $M_{2}(\Gamma_0(21))$	4

Decomposition of the old subspace

The old subspace is 0.

Provenance

Stored in the atlas: computed by Sagebrush's modular symbols engine in 0.103 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, 7, 21]: [0, 1, 1, 4] (certified at ell = 2147483647)
T = [(2, 1), (5, 1)]; h of degree 1 = 1 x 1 (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(21, 2)

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

cargo run --release -p sagebrush-web --example atlas -- 21:2 1000

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