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A264776 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is pentagonal: (3n^2 - n)/2. 6
1, 2, 47, 160, 6070026, 47418729166667, 4741872916666741666666666667, 47418729166667416666666666674166666666666666666666666667 (list; graph; refs; listen; history; text; internal format)
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 A304725 A128822

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

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Last modified June 20 09:27 EDT 2019. Contains 324234 sequences. (Running on oeis4.)