A101428 - OEIS

A101428

Number of ways to write n as an ordered sum of a triangular number (A000217) and a square (A000290).

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

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OFFSET

0,2

COMMENTS

0 is both a triangular number and a square number.

First occurrence of k beginning at 0: 8, 2, 1, 10, 37, 136, 235, 1549, 631, 2314, 2116, 11026, 3997, 148240, 19045, 20827, 25876, 893116, 67951, ?19?, 35974, 187444, 1542655, 354061, 131905, ?25?, ?26?, 835399, 323767, ?29?, 611560, ?31?, 515629, ?33?, ?34?, ?35?, 1187146, ?37?, ?38?, ?39?, 1474939, ..., . - Robert G. Wilson v, Mar 30 2014

Variant of A082660 (which allows only positive triangular numbers). - R. J. Mathar, Apr 28 2020

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000

FORMULA

G.f.: sum(i>=0, x^(i^2) ) * sum(i>=0, x^(i*(i+1)/2) ). - Ralf Stephan, May 17 2014

EXAMPLE

Examples: n=1 gives the a(1)=2 cases 1=1+0=0+1; a(26)=2 because 26=25+1=16+10.

MAPLE

A000217 := proc(n) n*(n+1)/2 ; end:

A101428 := proc(n)

local a, y, t ;

a := 0 ;

for y from 0 do

t := A000217(y) ;

if n-t < 0 then

RETURN(a) ;

else

if issqr(n-t) then

a := a+1 ;

fi;

od:

end:

for n from 0 to 100 do printf("%a, ", A101428(n)) ; od:

MATHEMATICA

t = FoldList[#1 + #2 &, 0, Range@ 15]; s = Range[0, 10]^2, a = Sort@ Flatten@ Table[ s[[j]] + t[[k]], {j, 15}, {k, 11}]; Table[Count[a, n], {n, 0, 104}] (* or *)

triQ[n_] := IntegerQ@ Sqrt[8n + 1]; f[n_] := Block[{c = k = 0, lmt = 2 + Floor[Sqrt[n]]}, While[k < lmt, If[ triQ[n - k^2], c++]; k++]; c]; Array[f, 105, 0] (* Robert G. Wilson v, Mar 30 2014 *)

CROSSREFS

Cf. A000925, A115171, A115172, A115173, A115174, A115175, A115176, A115177, A144642.

Sequence in context: A193805 A159704 A348194 * A360171 A348528 A307223

Adjacent sequences: A101425 A101426 A101427 * A101429 A101430 A101431

KEYWORD

nonn

AUTHOR

R. J. Mathar, Jan 03 2009

STATUS

approved