cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-9 of 9 results.

A052343 Number of ways to write n as the unordered sum of two triangular numbers (zero allowed).

Original entry on oeis.org

1, 1, 1, 1, 1, 0, 2, 1, 0, 1, 1, 1, 1, 1, 0, 1, 2, 0, 1, 0, 1, 2, 1, 0, 1, 1, 0, 1, 1, 1, 1, 2, 0, 0, 1, 0, 2, 1, 1, 1, 0, 0, 2, 1, 0, 1, 2, 0, 1, 1, 0, 2, 0, 0, 0, 2, 2, 1, 1, 0, 1, 1, 0, 0, 1, 1, 2, 1, 0, 1, 1, 0, 2, 1, 0, 0, 2, 0, 1, 1, 0, 3, 0, 1, 1, 0, 0, 1, 1, 0, 1, 2, 1, 1, 2, 0, 0, 1, 0, 1, 1, 1
Offset: 0

Views

Author

Christian G. Bower, Jan 23 2000

Keywords

Comments

Number of ways of writing n as a sum of a square and twice a triangular number (zeros allowed). - Michael Somos, Aug 18 2003
a(A020757(n))=0; a(A020756(n))>0; a(A119345(n))=1; a(A118139(n))>1. - Reinhard Zumkeller, May 15 2006
Also, number of ways to write 4n+1 as the unordered sum of two squares of nonnegative integers. - Vladimir Shevelev, Jan 21 2009
The average value of a(n) for n <= x is Pi/4 + O(1/sqrt(x)). - Vladimir Shevelev, Feb 06 2009

Examples

			G.f. = 1 + x + x^2 + x^3 + x^4 + 2*x^6 + x^7 + x^9 + x^10 + x^11 + ...
		

Crossrefs

Programs

  • Haskell
    a052343 = (flip div 2) . (+ 1) . a008441
    -- Reinhard Zumkeller, Jul 25 2014
  • Maple
    A052343 := proc(n)
        local a,t1idx,t2idx,t1,t2;
        a := 0 ;
        for t1idx from 0 do
            t1 := A000217(t1idx) ;
            if t1 > n then
                break;
            end if;
            for t2idx from t1idx do
                t2 := A000217(t2idx) ;
                if t1+t2 > n then
                    break;
                elif t1+t2 = n then
                    a := a+1 ;
                end if;
            end do:
        end do:
        a ;
    end proc: # R. J. Mathar, Apr 28 2020
  • Mathematica
    Length[PowersRepresentations[4 # + 1, 2, 2]] & /@ Range[0, 101] (* Ant King, Dec 01 2010 *)
    d1[k_]:=Length[Select[Divisors[k],Mod[#,4]==1&]];d3[k_]:=Length[Select[Divisors[k],Mod[#,4]==3&]];f[k_]:=d1[k]-d3[k];g[k_]:=If[IntegerQ[Sqrt[4k+1]],1/2 (f[4k+1]+1),1/2 f[4k+1]];g[#]&/@Range[0,101] (* Ant King, Dec 01 2010 *)
    a[ n_] := Length @ Select[ Table[ Sqrt[n - i - i^2], {i, 0, Quotient[ Sqrt[4 n + 1] - 1, 2]}], IntegerQ]; (* Michael Somos, Jul 28 2015 *)
    a[ n_] := Length @ FindInstance[ {j >= 0, k >= 0, j^2 + k^2 + k == n}, {k, j}, Integers, 10^9]; (* Michael Somos, Jul 28 2015 *)
  • PARI
    {a(n) = if( n<0, 0, sum(i=0, (sqrtint(4*n + 1) - 1)\2, issquare(n - i - i^2)))}; /* Michael Somos, Aug 18 2003 */
    

Formula

a(n) = ceiling(A008441(n)/2). - Reinhard Zumkeller, Nov 03 2009
G.f.: (Sum_{k>=0} x^(k^2 + k)) * (Sum_{k>=0} x^(k^2)). - Michael Somos, Aug 18 2003
Recurrence: a(n) = Sum_{k=1..r(n)} r(2n-k^2+k) - C(r(n),2) - a(n-1) - a(n-2) - ... - a(0), n>=1,a (0)=1, where r(n)=A000194(n+1) is the nearest integer to square root of n+1. For example, since r(6)=3, a(6) = r(12) + r(10) + r(6) - C(3,2) - a(5) - ... - a(0) = 4 + 3 + 3 - 3 - 0 - 1 - 1 - 1 - 1 - 1 = 2. - Vladimir Shevelev, Feb 06 2009
a(n) = A025426(8n+2). - Max Alekseyev, Mar 09 2009
a(n) = (A002654(4n+1) + A010052(4n+1)) / 2. - Ant King, Dec 01 2010
a(2*n + 1) = A053692(n). a(4*n + 1) = A259287(n). a(4*n + 3) = A259285(n). a(6*n + 1) = A260415(n). a(6*n + 4) = A260516(n). - Michael Somos, Jul 28 2015
a(3*n) = A093518(n). a(3*n + 1) = A121444(n). a(9*n + 2) = a(n). a(9*n + 5) = a(9*n + 8) = 0. - Michael Somos, Jul 28 2015
Convolution of A005369 and A010052. - Michael Somos, Jul 28 2015

A052344 Number of ways to write n as the unordered sum of two nonzero triangular numbers.

Original entry on oeis.org

0, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 2, 0, 1, 0, 1, 1, 1, 0, 1, 1, 0, 1, 0, 1, 1, 2, 0, 0, 1, 0, 1, 1, 1, 1, 0, 0, 2, 1, 0, 0, 2, 0, 1, 1, 0, 2, 0, 0, 0, 1, 2, 1, 1, 0, 1, 1, 0, 0, 1, 1, 1, 1, 0, 1, 1, 0, 2, 1, 0, 0, 2, 0, 0, 1, 0, 3, 0, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1, 2, 0, 0, 1, 0, 1, 1, 1
Offset: 0

Views

Author

Christian G. Bower, Jan 23 2000

Keywords

Comments

Number of ways to write 8*n+2 as the unordered sum of two odd squares > 1. - Robert Israel, Feb 24 2016
Number of partitions of 2n into two promic numbers > 1. - Wesley Ivan Hurt, Jun 09 2021

Crossrefs

Programs

  • Maple
    G:= (1/8)*(JacobiTheta2(0, sqrt(q))^2-4*JacobiTheta2(0, sqrt(q))*q^(1/8)+2*JacobiTheta2(0, q))/q^(1/4):
    S:= series(G,q,1001):
    seq(coeff(S,q,j),j=0..1000); # Robert Israel, Feb 24 2016
  • Mathematica
    nn=150; tri=Accumulate[Range[nn]]; t=Table[0, {tri[[-1]]}]; Do[n=tri[[i]]+tri[[j]]; If[n <= tri[[-1]], t[[n]]++], {i,nn}, {j,i}]; t=Prepend[t,0]

Formula

G.f.: (Theta_2(sqrt(x))^2 - 4*x^(1/8)*Theta_2(sqrt(x)) + 2*Theta_2(x))/(8*x^(1/4)) where Theta_2 is a Jacobi theta function. - Robert Israel, Feb 24 2016
a(n) = Sum_{k=1..n} c(k) * c(2*n-k), where c(n) is the characteristic function of promic numbers (A005369). - Wesley Ivan Hurt, Jun 09 2021
a(n) = Sum_{k=1..floor(n/2)} c(k) * c(n-k), where c = A010054. - Wesley Ivan Hurt, Jan 06 2024

A052346 Smallest number which is the sum of two positive triangular numbers in exactly n different ways.

Original entry on oeis.org

1, 2, 16, 81, 471, 1056, 1381, 11781, 6906, 17956, 34531, 123256, 40056, 4462656, 305256, 448906, 200281, 1957231, 520731, 10563906, 1001406, 11222656, 539550781, 3454506, 1482081, 75865156, 7172606106, 8852431, 25035156, 334020781, 13018281, 38531031, 7410406, 7014160156
Offset: 0

Views

Author

Christian G. Bower, Jan 23 2000

Keywords

Comments

From Chai Wah Wu, Oct 20 2023: (Start)
Other terms:
a(35) = 42980356
a(36) = 19267056
a(38) = 1289707656
a(39) = 2782318906
a(40) = 37052031
a(41) = 256720506
a(42) = 325457031
a(45) = 221310781
a(47) = 550240551
a(48) = 60765331
a(50) = 2200089531
a(54) = 327539956
a(56) = 926300781
a(59) = 7629645156
a(60) = 481676406
a(63) = 4598740656
a(64) = 303826656
a(68) = 6418012656
a(71) = 4579579956
a(72) = 789949306
a(80) = 1519133281
a(81) = 9498658731
a(84) = 12041910156
a(90) = 8188498906
a(96) = 3220562556
a(108) = 13429138206
(End)

Examples

			a(4) = 471 because 471 is the sum of two positive triangular numbers in exactly 4 different ways (as 300+171, 351+120, 435+36, and 465 + 6), and there is no smaller number that has this property.
		

Crossrefs

Extensions

a(27), a(28) = 8852431, 25035156; a(26) not yet found
a(26) from Donovan Johnson, Nov 17 2008
Name edited (added the qualifier "positive"), example edited, and a(29)-a(32) added by Jon E. Schoenfield, Jul 16 2017
a(33) from Chai Wah Wu, Oct 20 2023

A052348 Indices of A052343 (ways to write n as sum of two triangular numbers) where record values are reached.

Original entry on oeis.org

0, 6, 81, 276, 1056, 1381, 6906, 17956, 34531, 40056, 200281, 520731, 1001406, 1482081, 7410406, 19267056, 37052031, 60765331, 303826656, 789949306, 1519133281, 3220562556, 13429138206, 16102812781, 41867313231, 80514063906, 196454315931, 711744324931
Offset: 1

Views

Author

Christian G. Bower, Jan 23 2000

Keywords

Examples

			a(2) = 6 because 6 = 6 + 0 = 3 + 3. a(3) = 81 because 81 = 3 + 78 = 15 + 66 = 36 + 55.
		

Crossrefs

Probably differs from A053587 only at n=1, 2, 4.

Extensions

a(23)-a(26) from Donovan Johnson, May 24 2009
a(27)-a(28) from Donovan Johnson, Mar 20 2013

A334013 a(n) is the least integer that is the sum of two nonzero tetrahedral numbers in exactly n ways.

Original entry on oeis.org

2, 140, 102424, 43322844421220
Offset: 1

Views

Author

Ilya Gutkovskiy, Apr 12 2020

Keywords

Examples

			Let Te(k) denote the k-th tetrahedral number, then
a(1) = Te(1) + Te(1);
a(2) = Te(7) + Te(6) = Te(8) + Te(4);
a(3) = Te(82) + Te(34) = Te(83) + Te(27) = Te(84) + Te(7);
a(4) = Te(60500) + Te(33760) = Te(53311) + Te(47682) = Te(63383) + Te(17423) = Te(63495) + Te(15790);
		

Crossrefs

Extensions

a(4) from Giovanni Resta, Apr 13 2020

A052347 Record values reached in A052343 and A052344.

Original entry on oeis.org

1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 16, 18, 20, 24, 32, 36, 40, 48, 64, 72, 80, 96, 108, 128, 144, 160, 192, 216
Offset: 1

Views

Author

Christian G. Bower, Jan 23 2000

Keywords

Crossrefs

Extensions

a(25)-a(26) from Donovan Johnson, Jun 26 2010
a(27)-a(28) from Donovan Johnson, Mar 20 2013

A052345 Least k such that A052343(k)=n.

Original entry on oeis.org

5, 1, 6, 81, 276, 1056, 1381, 50781, 6906, 17956, 34531, 660156, 40056, 4462656, 305256, 448906, 200281, 412597656, 520731, 12397766113281, 1001406, 11222656, 539550781, 7631406, 1482081, 75865156, 422394133, 8852431, 25035156, 161170959472656
Offset: 0

Views

Author

Christian G. Bower, Jan 23 2000

Keywords

Crossrefs

Extensions

a(19) and a(29) from Max Alekseyev, Mar 09 2009
a(19) corrected by Max Alekseyev, Mar 11 2009

A234004 Smallest number which is the sum of two positive triangular numbers (A000217) in at least n ways.

Original entry on oeis.org

2, 16, 81, 471, 1056, 1381, 6906, 6906, 17956, 34531, 40056, 40056, 200281, 200281, 200281, 200281, 520731, 520731, 1001406, 1001406, 1482081, 1482081, 1482081, 1482081, 7410406, 7410406, 7410406, 7410406, 7410406, 7410406, 7410406, 7410406
Offset: 1

Views

Author

Keywords

Examples

			2 = 1+1; 16 = 1+15 = 6+10; 81 = 3+78 = 15+66 = 36+45.
		

Crossrefs

Programs

A334077 a(n) is the smallest positive integer that can be expressed as the difference of two positive triangular numbers in at least n ways.

Original entry on oeis.org

2, 5, 9, 27, 45, 63, 105, 135, 225, 315, 315, 315, 945, 945, 945, 945, 1575, 1575, 2835, 2835, 3465, 3465, 3465, 3465, 10395, 10395, 10395, 10395, 10395, 10395, 10395, 10395, 17325, 17325, 17325, 17325, 31185, 31185, 31185, 31185, 45045, 45045, 45045, 45045
Offset: 1

Views

Author

Ilya Gutkovskiy, Apr 13 2020

Keywords

Crossrefs

Showing 1-9 of 9 results.