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A030666 Smallest nontrivial extension of n which is a square. 5
16, 25, 36, 49, 529, 64, 729, 81, 900, 100, 1156, 121, 1369, 144, 1521, 169, 1764, 1849, 196, 2025, 2116, 225, 2304, 2401, 256, 2601, 2704, 289, 2916, 3025, 3136, 324, 3364, 3481, 35344, 361, 3721, 3844, 3969, 400, 41209, 4225, 4356, 441 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A trivial extension would mean appending no digits at all when n is already a square. With trivial extensions allowed, this sequence becomes A018796.

LINKS

N. J. A. Sloane, Table of n, a(n) for n = 1..20000

FORMULA

a(n) = A030667(n)^2.

EXAMPLE

80 is not a perfect square, but 81 = 9^2, so a(8) = 81.

Although 9 is already a square, we need to append at least one digit. However, none of 90, 91, 92, ..., 99 are squares. Then we try 900 = 30^2, so a(9) = 900.

MAPLE

# Program which computes 20000 terms, from N. J. A. Sloane, Nov 24 2015

for b from 1 to 20000 do

sw1:=-1:

for p from 1 to 6 do

bp:=b*10^p;

for i from 0 to 10^p-1 do

if issqr(bp+i) then lprint(b, bp+i); sw1:=1; break; fi;

                       od:

if sw1 > 0 then break; fi;

                 od:

if sw1 < 0 then lprint("failed at b = ", b); fi;

                      od:

PROG

(Python)

from gmpy2 import isqrt

def A030666(n):

    d, nd = 10, 10*n

    while True:

        x = (isqrt(nd-1)+1)**2

        if x < nd+d:

            return int(x)

        d *= 10

        nd *= 10 # Chai Wah Wu, May 24 2016

CROSSREFS

Cf. A030667, A030686, A018796.

See A264604 for another version (first differs at a(9)).

Sequence in context: A319388 A291334 A175689 * A030676 A264604 A218439

Adjacent sequences:  A030663 A030664 A030665 * A030667 A030668 A030669

KEYWORD

nonn,base

AUTHOR

Patrick De Geest

STATUS

approved

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Last modified June 4 08:18 EDT 2020. Contains 334825 sequences. (Running on oeis4.)