A006203 -id:A006203 - cp's OEIS frontend

A046015 Discriminants of imaginary quadratic fields with class number 18 (negated).

335, 519, 527, 679, 1135, 1172, 1207, 1383, 1448, 1687, 1691, 1927, 2047, 2051, 2167, 2228, 2291, 2315, 2344, 2644, 2747, 2859, 3035, 3107, 3543, 3544, 3651, 3688, 4072, 4299, 4307, 4568, 4819, 4883, 5224, 5315, 5464, 5492, 5539, 5899

Offset: 1

Eric W. Weisstein

The class group of Q[sqrt(-d)] is isomorphic to C_3 X C_6 for d = 9748, 12067, 16627, 17131, 19651, 22443, 23683, 34027, 34507. For all other known d in this sequence, the class group of Q[sqrt(-d)] is isomorphic to C_18. - Jianing Song, Dec 01 2019

Jianing Song, Table of n, a(n) for n = 1..150
Steven Arno, M. L. Robinson and Ferrel S. Wheeler, Imaginary quadratic fields with small odd class number, Acta Arithm. 83.4 (1998), 295-330
Duncan A. Buell, Small class numbers and extreme values of L-functions of quadratic fields, Math. Comp., 31 (1977), 786-796.
C. Wagner, Class Number 5, 6 and 7, Math. Comput. 65, 785-800, 1996.
Eric Weisstein's World of Mathematics, Class Number.
Index entries for sequences related to quadratic fields

Cf. A006203, A013658, A014602, A014603, A046002-A046020.

Reap[ For[n = 1, n < 6000, n++, s = Sqrt[-n]; If[ NumberFieldClassNumber[s] == 18, d = -NumberFieldDiscriminant[s]; Print[d]; Sow[d]]]][[2, 1]] // Union (* Jean-François Alcover, Oct 05 2012 *)

A107628 Number of integral quadratic forms ax^2 + bxy + cy^2 whose discriminant b^2-4ac is -n, 0 <= b <= a <= c and gcd(a,b,c) = 1.

Original entry on oeis.org

0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 0, 2, 1, 0, 0, 1, 2, 0, 0, 2, 2, 0, 0, 1, 1, 0, 0, 2, 2, 0, 0, 2, 2, 0, 0, 3, 2, 0, 0, 1, 2, 0, 0, 3, 2, 0, 0, 2, 2, 0, 0, 3, 3, 0, 0, 2, 2, 0, 0, 3, 2, 0, 0, 1, 3, 0, 0, 4, 2, 0, 0, 2, 2, 0, 0, 3, 3, 0, 0, 2, 4, 0, 0, 4, 2, 0, 0, 2, 2, 0, 0, 5, 4, 0, 0, 2, 2, 0, 0, 3, 4, 0

Offset: 1

T. D. Noe, May 18 2005, Apr 30 2008

nonn

This sequence is closely related to the class number function, h(-n), which is given for fundamental discriminants in A006641. For a fundamental discriminant d, we have h(-d) < 2a(d). It appears that a(n) < Sqrt(n) for all n. For k>1, the primes p for which a(p)=k coincide with the numbers n such that the class number h(-n) is 2k-1 (see A006203, A046002, A046004, A046006. A046008, A046010, A046012, A046014, A046016 A046018, A046020). - T. D. Noe, May 07 2008

			a(15)=2 because the forms x^2 + xy + 4y^2 and 2x^2 + xy + 2y^2 have discriminant -15.

See A106856.

T. D. Noe, Table of n, a(n) for n=1..10000

Cf. A106856 (start of many quadratic forms).
Cf. A133675 (n such that a(n)=1).
Cf. A223708 (without zeros).

dLim=150; cnt=Table[0, {dLim}]; nn=Ceiling[dLim/4]; Do[d=b^2-4a*c; If[GCD[a, b, c]==1 && 0<-d<=dLim, cnt[[ -d]]++ ], {b, 0, nn}, {a, b, nn}, {c, a, nn}]; cnt

{a(n)=local(m); if(n<3, 0, forvec(v=vector(3,k,[0,(n+1)\4]), if( (gcd(v)==1)&(-v[1]^2+4*v[2]*v[3]==n), m++ ), 1); m)} /* Michael Somos, May 31 2005 */

A002149 Largest prime p==3 (mod 8) such that Q(sqrt(-p)) has class number 2n+1.

Original entry on oeis.org

163, 907, 2683, 5923, 10627, 15667, 20563, 34483, 37123, 38707, 61483, 90787, 93307, 103387, 166147, 133387, 222643, 210907, 158923, 253507, 296587

Offset: 0

N. J. A. Sloane

nonn

Most of these values are only conjectured to be correct.
Apr 15 2008: David Broadhurst says: I computed class numbers for prime discriminants with |D| < 10^9, but stopped when the first case with |D| > 5*10^8 was observed. That factor of 2 seems to me to be a reasonable margin of error, when you look at the pattern of what is included.
Arno, Robinson, & Wheeler prove a(0)-a(11). - Charles R Greathouse IV, Apr 25 2013

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

David Broadhurst, Table of n, a(n) for n = 0..739 (conjectural; see comment)
Steven Arno, M. L. Robinson, and Ferrell S. Wheeler, Imaginary quadratic fields with small odd class number, Acta Arith. 83 (1998), pp. 295-330.
D. Shanks, Review of R. B. Lakein and S. Kuroda, Tables of class numbers h(-p) for fields Q(sqrt(-p)), p <= 465071, Math. Comp., 24 (1970), 491-492.

Cf. A002148, A003173, A006203.

Edited by Dean Hickerson, Mar 17 2003

A123563 Discriminants of imaginary quadratic fields with class number 20 (negated).

Original entry on oeis.org

455, 615, 776, 824, 836, 920, 1064, 1124, 1160, 1263, 1284, 1460, 1495, 1524, 1544, 1592, 1604, 1652, 1695, 1739, 1748, 1796, 1880, 1887, 1896, 1928, 1940, 1956, 2136, 2247, 2360, 2404, 2407, 2483, 2487, 2532, 2552

Offset: 1

Eric W. Weisstein, Nov 19 2006

A finite sequence with exactly 350 terms.

Eric Weisstein's World of Mathematics, Class Number.

Cf. A006203, A013658, A014602, A014603, A046002, ..., A046016, A046018, A046020.

Reap[ For[n = 1, n < 3000, n++, s = Sqrt[-n]; If[ NumberFieldClassNumber[s] == 20, d = -NumberFieldDiscriminant[s]; Print[d]; Sow[d]]]][[2, 1]] // Union (* Jean-François Alcover, Oct 05 2012 *)

A351666 Discriminants of imaginary quadratic fields with class number 28 (negated).

Original entry on oeis.org

831, 935, 1095, 1311, 1335, 1364, 1455, 1479, 1496, 1623, 1703, 1711, 1855, 1976, 2024, 2055, 2120, 2127, 2324, 2359, 2431, 2455, 2564, 2607, 2616, 2703, 3224, 3272, 3396, 3419, 3487, 3535, 3572, 3576, 3608, 3624, 3731, 3848, 3995, 4040, 4183, 4279, 4344

Offset: 1

Andy Huchala, Feb 16 2022

Sequence contains 457 terms; largest is 126043.

The class groups associated to 174 of the above discriminants are isomorphic to C_28, and the remaining 283 have a class group isomorphic to C_14 X C_2.

Andy Huchala, Table of n, a(n) for n = 1..457
Eric Weisstein's World of Mathematics, Class Number

Cf. A006203, A013658, A014602, A014603, A046002-A046020, A046125, A056987, A351664.

isok(n) = {isfundamental(-n) && quadclassunit(-n).no == 28}; \\ Michel Marcus, Mar 02 2022

ls = [(QuadraticField(-n, 'a').discriminant(), QuadraticField(-n, 'a').class_number()) for n in (0..10000) if is_fundamental_discriminant(-n) and not is_square(n)];
[-a[0] for a in ls if a[1] == 28]

A351679 Discriminants of imaginary quadratic fields with class number 41 (negated).

Original entry on oeis.org

1151, 2551, 2719, 3079, 3319, 3511, 6143, 9319, 9467, 10499, 10903, 11047, 11483, 11719, 11987, 12227, 12611, 13567, 14051, 14411, 14887, 14983, 16067, 16187, 19763, 20407, 20771, 21487, 22651, 24971, 25171, 26891, 26987, 27739, 28547, 29059, 29251, 30859

Offset: 1

Andy Huchala, Mar 28 2022

Sequence contains 109 terms; largest is 296587.

The class group of Q[sqrt(-d)] is isomorphic to C_41 for all d in this sequence.

Andy Huchala, Table of n, a(n) for n = 1..109
Mark Watkins, Class numbers of imaginary quadratic fields, Mathematics of Computation, 73, pp. 907-938.
Eric Weisstein's World of Mathematics, Class Number

Cf. A006203, A013658, A014602, A014603, A046002-A046020, A046125, A056987, A351664-A351680.

ls = [(QuadraticField(-n, 'a').discriminant(), QuadraticField(-n, 'a').class_number()) for n in (0..10000) if is_fundamental_discriminant(-n) and not is_square(n)];
[-a[0] for a in ls if a[1] == 41]

A351665 Discriminants of imaginary quadratic fields with class number 27 (negated).

Original entry on oeis.org

983, 1231, 1399, 1607, 1759, 1879, 1999, 3271, 3299, 3943, 4903, 6007, 6011, 7699, 8867, 10531, 10939, 11003, 11027, 11383, 11491, 11779, 11939, 13411, 14243, 14723, 15107, 15739, 16411, 16547, 17443, 17627, 17659, 17747, 18587, 18787, 18859, 19051, 19427

Offset: 1

Andy Huchala, Feb 16 2022

Sequence contains 93 terms; largest is 103387.

The class group of Q[sqrt(-d)] is isomorphic to C_9 X C_3 for d = 3299, 19427, 34603, 89923, and 98443. For all other d in this sequence, the class group of Q[sqrt(-d)] is isomorphic to C_27.

Andy Huchala, Table of n, a(n) for n = 1..93
Eric Weisstein's World of Mathematics, Class Number

Cf. A006203, A013658, A014602, A014603, A046002-A046020, A046125, A056987, A351664.

isok(n) = {isfundamental(-n) && quadclassunit(-n).no == 27}; \\ Michel Marcus, Mar 02 2022

ls = [(QuadraticField(-n, 'a').discriminant(), QuadraticField(-n, 'a').class_number()) for n in (0..10000) if is_fundamental_discriminant(-n) and not is_square(n)];
[-a[0] for a in ls if a[1] == 27]

A351667 Discriminants of imaginary quadratic fields with class number 29 (negated).

Original entry on oeis.org

887, 2287, 2311, 2383, 2939, 3583, 3659, 3823, 4451, 4519, 5051, 5743, 6947, 7207, 7643, 7687, 8863, 8963, 9323, 12323, 13763, 13883, 14387, 15139, 15227, 15443, 15467, 15859, 16427, 17491, 20483, 20507, 22051, 23059, 23251, 24859, 25523, 28403, 29587, 29723

Offset: 1

Andy Huchala, Mar 24 2022

Sequence contains 83 terms; largest is 166147.

The class group of Q[sqrt(-d)] is isomorphic to C_29 for all d in this sequence.

Andy Huchala, Table of n, a(n) for n = 1..83
Eric Weisstein's World of Mathematics, Class Number

Cf. A006203, A013658, A014602, A014603, A046002-A046020, A046125, A056987, A351664-A351666.

ls = [(QuadraticField(-n, 'a').discriminant(), QuadraticField(-n, 'a').class_number()) for n in (0..10000) if is_fundamental_discriminant(-n) and not is_square(n)];
[-a[0] for a in ls if a[1] == 29]

A351668 Discriminants of imaginary quadratic fields with class number 30 (negated).

Original entry on oeis.org

671, 815, 1007, 1844, 2036, 2071, 2191, 2264, 2319, 2599, 2708, 3188, 3223, 3284, 3439, 3991, 4087, 4276, 4696, 4835, 4859, 4979, 5579, 5912, 6107, 6459, 6463, 6488, 6535, 6635, 7087, 7115, 7303, 7576, 7835, 7971, 8259, 8267, 8367, 8483, 8948, 9019, 9076

Offset: 1

Andy Huchala, Mar 24 2022

Sequence contains 255 terms; largest is 134467.

The class group of Q[sqrt(-d)] is isomorphic to C_30 for all d in this sequence.

Andy Huchala, Table of n, a(n) for n = 1..255
Eric Weisstein's World of Mathematics, Class Number

Cf. A006203, A013658, A014602, A014603, A046002-A046020, A046125, A056987, A351664.

ls = [(QuadraticField(-n, 'a').discriminant(), QuadraticField(-n, 'a').class_number()) for n in (0..10000) if is_fundamental_discriminant(-n) and not is_square(n)];
[-a[0] for a in ls if a[1] == 30]

A351669 Discriminants of imaginary quadratic fields with class number 31 (negated).

Original entry on oeis.org

719, 911, 2927, 3251, 3727, 3779, 4159, 4951, 5651, 6131, 6491, 7639, 8647, 9203, 10427, 11863, 12347, 12923, 13043, 13219, 13687, 14627, 14731, 15923, 17987, 18803, 19219, 20611, 24691, 24979, 28051, 32083, 32363, 35491, 38851, 39667, 39883, 41227, 41539

Offset: 1

Andy Huchala, Mar 24 2022

Sequence contains 73 terms; largest is 133387.

The class group of Q[sqrt(-d)] is isomorphic to C_31 for all d in this sequence.

Andy Huchala, Table of n, a(n) for n = 1..73
Eric Weisstein's World of Mathematics, Class Number

Cf. A006203, A013658, A014602, A014603, A046002-A046020, A046125, A056987, A351664.

ls = [(QuadraticField(-n, 'a').discriminant(), QuadraticField(-n, 'a').class_number()) for n in (0..10000) if is_fundamental_discriminant(-n) and not is_square(n)];
[-a[0] for a in ls if a[1] == 31]

cp's OEIS Frontend

A046015 Discriminants of imaginary quadratic fields with class number 18 (negated).

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A107628 Number of integral quadratic forms ax^2 + bxy + cy^2 whose discriminant b^2-4ac is -n, 0 <= b <= a <= c and gcd(a,b,c) = 1.

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A002149 Largest prime p==3 (mod 8) such that Q(sqrt(-p)) has class number 2n+1.

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A123563 Discriminants of imaginary quadratic fields with class number 20 (negated).

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A351666 Discriminants of imaginary quadratic fields with class number 28 (negated).

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A351679 Discriminants of imaginary quadratic fields with class number 41 (negated).

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A351665 Discriminants of imaginary quadratic fields with class number 27 (negated).

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Sage

A351667 Discriminants of imaginary quadratic fields with class number 29 (negated).

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Sage

A351668 Discriminants of imaginary quadratic fields with class number 30 (negated).

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