Find this integer with these properties.

What is the smallest positive integer n which CANNOT be written in any of the following forms ?

• n = 1 + 2 + · · · + k for a positive integer k.

• n = p^k , for a prime number p and integer k.

• n = p + 1 for a prime number p


The answer is 22.

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1 solution

Matteo Damiano
Sep 30, 2018

To find the smallest integer n n with these properties, it is enough to check that every positive integer up to n 1 n-1 does not satisfy all the conditions.

1 = 2 0 1=2^0 , condition 2

2 = 2 1 2=2^1 , condition 2

3 = 3 1 3=3^1 , condition 2

4 = 3 + 1 4=3+1 , condition 3

5 = 5 1 5=5^1 , condition 2

6 = 5 + 1 6=5+1 , condition 3

7 = 7 1 7=7^1 , condition 2

8 = 2 3 8=2^3 , condition 2

9 = 3 2 9=3^2 , condition 2

10 = 1 + 2 + 3 + 4 10=1+2+3+4 , condition 1

11 = 1 1 1 11=11^1 , condition 2

12 = 11 + 1 12=11+1 , condition 3

13 = 1 3 1 13=13^1 , condition 2

14 = 13 + 1 14=13+1 , condition 3

15 = 1 + 2 + 3 + 4 + 5 15=1+2+3+4+5 , condition 1

16 = 2 4 16=2^4 , condition 2

17 = 1 7 1 17=17^1 , condition 2

18 = 17 + 1 18=17+1 , condition 3

19 = 1 9 1 19=19^1 , condition 2

20 = 19 + 1 20=19+1 , condition 3

21 = 1 + 2 + 3 + 4 + 5 + 6 21=1+2+3+4+5+6 , condition 1

22 22 isn't the sum of the first k integers, because when k = 6 k=6 the sum is 21 21 and when k = 7 k=7 the sum is 28 28 . It's not a power of a prime, either, because it is the product of two distinct primes, and it's not 1 1 more than a prime, because 21 21 is composite.

The correct answer is 22 \boxed{22}

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