cp's OEIS Frontend

A100878 Smallest number of pentagonal numbers which sum to n.

%I A100878 #64 Jul 18 2022 20:18:36
%S A100878 0,1,2,3,4,1,2,3,4,5,2,3,1,2,3,3,4,2,3,4,4,5,1,2,2,3,4,2,3,3,4,5,3,4,
%T A100878 2,1,2,3,4,3,2,3,4,5,2,3,3,2,3,3,4,1,2,3,4,5,2,2,3,3,4,3,3,2,3,4,3,4,
%U A100878 3,3,1,2,3,2,3,2,3,4,3,3,3,4,2,3,4,3,2,3,4,5,4,3,1,2,3,3,4,2,3,4,4,4,2,3,2
%N A100878 Smallest number of pentagonal numbers which sum to n.
%C A100878 From _Bernard Schott_, Jul 15 2022: (Start)
%C A100878 In September 1636, Fermat, in a letter to Mersenne, made the statement that every number is a sum of at most three triangular numbers, four squares, five pentagonal numbers, and so on.
%C A100878 The square case was proved by Lagrange in 1770; it is known as Lagrange's four squares theorem (see A002828). Then Gauss proved the triangular case in 1796 (see A061336).
%C A100878 In 1813, Cauchy proved this polygonal number theorem: for m >= 3, every positive integer N can be represented as a sum of m+2 (m+2)-gonal numbers, at most four of which are different from 0 and 1 (Deza reference). Hence every number is expressible as the sum of at most five positive pentagonal numbers (A000326). (End)
%D A100878 Elena Deza and Michel Marie Deza, Fermat's polygonal number theorem, Figurate numbers, World Scientific Publishing (2012), Chapter 5, pp. 313-377.
%D A100878 Richard K. Guy, Unsolved Problems in Number Theory, 3rd Edition, Springer, 2004, Section D3, Figurate numbers, pp. 222-228.
%H A100878 Augustin-Louis Cauchy, <a href="https://doi.org/10.1017/CBO9780511702655.028">Démonstration du théorème général de Fermat sur les nombres polygones</a>, Extrait des Mémoires de l'Institut, 1813-15.
%H A100878 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatsPolygonalNumberTheorem.html">Fermat's Polygonal Number Theorem</a>.
%H A100878 Wikipedia, <a href="https://en.wikipedia.org/wiki/Fermat_polygonal_number_theorem">Fermat polygonal number theorem</a>.
%F A100878 a(n) <= 5 (inequality proposed by Fermat and proved by Cauchy). - _Bernard Schott_, Jul 13 2022
%e A100878 a(5)=1 since 5=5, a(6)=2 since 6=1+5, a(7)=3 since 7=1+1+5, a(10)=2 since 10=5+5 with 1 and 5 pentagonal numbers.
%o A100878 (PARI) a(n) = my(nb=oo); forpart(vp=n, if (vecsum(apply(x->ispolygonal(x, 5), Vec(vp))) == #vp, nb = min(nb, #vp)),,5); nb; \\ _Michel Marcus_, Jul 15 2022
%o A100878 (PARI) a(n) = for(i = 1, oo, p = partitions(n, , [i,i]); for(j = 1, #p, if(sum(k = 1, i, ispolygonal(p[j][k],5)) == i, return(i)))) \\ _David A. Corneth_, Jul 15 2022
%Y A100878 Cf. A002828, A061336, A355717.
%Y A100878 Cf. A000326 (a(n) = 1), A003679 (a(n) = 4 or 5), A355660 (a(n) = 4), A133929 (a(n) = 5).
%K A100878 nonn
%O A100878 0,3
%A A100878 _Franz Vrabec_, Jan 09 2005
%E A100878 More terms from _David Wasserman_, Mar 04 2008