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A258660
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Numbers n such that the number of digits d in n is not prime and for each factor f of d the sum of the d/f digit groupings of size f is a square.
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2
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1, 4, 9, 1521, 3600, 7396, 8100, 103041, 120409, 160801, 11471769, 11655396, 12802084, 15210000, 22724289, 36000000, 42889401, 42928704, 45481536, 45968400, 46009089, 54567769, 61811044, 62236321, 70006689, 73925604, 73960000, 76965529, 79174404, 81000000, 85008400, 97693456, 97713225, 100000000
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listen;
history;
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internal format)
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OFFSET
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1,2
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COMMENTS
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If a(n) has m = p^k digits, then a(n)*10^((p-1)*m) is also a member of the sequence. For instance, 1521*10^(2^k-4) is in the sequence for all integers k >=2. # Chai Wah Wu, Jun 08 2015
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LINKS
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FORMULA
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PROG
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(Python)
from sympy import divisors
from gmpy2 import is_prime, isqrt, isqrt_rem, is_square
for l in range(1, 17):
....if not is_prime(l):
........fs = divisors(l)
........a, b = isqrt_rem(10**(l-1))
........if b > 0:
............a += 1
........for n in range(a, isqrt(10**l-1)+1):
............n2 = n**2
............ns = str(n2)
............for g in fs:
................y = 0
................for h in range(0, l, g):
....................y += int(ns[h:h+g])
................if not is_square(y):
....................break
............else:
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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