A264776 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is pentagonal: (3n^2 - n)/2.

1, 2, 47, 160, 6070026, 47418729166667, 4741872916666741666666666667, 47418729166667416666666666674166666666666666666666666667

Offset: 1

Author

Anders Hellström, Nov 24 2015

Keywords

• nonn
• base
• more

Comments

It appears that a(n) = ceiling((a(n-1) + 5/12)*10^(7*2^(n-6))) for n >= 7. - Jon E. Schoenfield, Nov 24 2015

Examples

			1, 12, 1247, 1247160, 12471606070026 are pentagonal.
		

Links

• Wikipedia, Pentagonal number

Crossrefs

Cf. A000326, A264738.

Programs

PARI

ispentagonal(n)=ispolygonal(n,5)
first(m)=my(v=vector(m),s="");s="1";v[1]=1;for(i=2,m,n=1;while(!ispentagonal(eval(concat(s,Str(n)))),n++);v[i]=n;s=concat(s,Str(n)));v

Extensions

a(6)-a(8) from Jon E. Schoenfield, Nov 24 2015