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A321291
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Smallest positive number for which the 4th power cannot be written as sum of 4th powers of any subset of previous terms.
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5
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1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 27, 28, 32, 34, 36, 38, 40, 42, 44, 46, 48, 52, 54, 56, 64, 68, 72, 76, 80, 84, 88, 92, 96, 104, 108, 112, 128, 136, 144, 152, 160, 168, 176, 184, 192, 208, 216, 224, 256
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OFFSET
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1,2
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COMMENTS
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a(n)^4 forms a sum-free sequence.
It is noteworthy that the terms of this sequence increase slower than those of similar sequences for smaller (A321266, A321290) but also larger powers (A321292, A321293).
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LINKS
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FORMULA
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a(n) = 2 * a(n-12) for n > 25 (conjectured).
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EXAMPLE
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The smallest number > 0 that is not in the sequence is 15, because
15^4 = 4^4 + 6^4 + 8^4 + 9^4 + 14^4.
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PROG
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(Python)
def findSum(nopt, tgt, a, smax, pwr):
....if nopt==0:
........return [] if tgt==0 else None
....if tgt<0 or tgt>smax[nopt-1]:
........return None
....rv=findSum(nopt-1, tgt - a[nopt-1]**pwr, a, smax, pwr)
....if rv!=None:
........rv.append(a[nopt-1])
....else:
........rv=findSum(nopt-1, tgt, a, smax, pwr)
....return rv
....POWER=4 ; x=0 ; a=[] ; smax=[] ; sumpwr=0
....while len(a)<n:
........while True:
............x+=1
............lst=findSum(len(a), x**POWER, a, smax, POWER)
............if lst==None:
................break
............rhs = " + ".join(["%d^%d"%(i, POWER) for i in lst])
............print(" %d^%d = %s"%(x, POWER, rhs))
........a.append(x) ; sumpwr+=x**POWER
........print("a(%d) = %d"%(len(a), x))
........smax.append(sumpwr)
....return a[-1]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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