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

1,2

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

Links

Table of n, a(n) for n=1..8.

Wikipedia, Pentagonal number

Example

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

Prog

(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

Crossrefs

Cf. A000326, A264738.

Sequence in context: A069548 A065044 A142313 * A153213 A355009 A304725

Adjacent sequences: A264773 A264774 A264775 * A264777 A264778 A264779

Keyword

nonn,base,more

Author

Anders Hellström, Nov 24 2015

Extensions

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

Status

approved