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A264776
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a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is pentagonal: (3n^2 - n)/2.
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6
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OFFSET
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1,2
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COMMENTS
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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
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LINKS
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EXAMPLE
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1, 12, 1247, 1247160, 12471606070026 are pentagonal.
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PROG
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(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
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CROSSREFS
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KEYWORD
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nonn,base,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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