2017 is a prime number

Happy New Year to you. 2017 is a prime number. The next time that the year will be a prime number is 2027. The last time it happened was 2011. I’ve worked these things out, so you don’t have to.

A recent question on “University Challenge” (a Christmas Special edition, where the contestants are alumni rather than current students) prompted me to check whether the year was prime. The question was, “What is the only single-digit number that is not a factor of 2016?” If the answer hasn’t come to you immediately, and you want to think about it for a moment, look away now.

It might seem obvious when you think about it, but for me it didn’t come to mind straight away, and the first person to buzz in on the show guessed wrong.

The answer is 5. Of course it is. 2016 doesn’t end in 5 or 0, so 5 cannot be one of its factors, but I now know that, as well as the obvious ones (1 and 2), 3, 4, 6, 7, 8 and 9 are all factors of 2016.

For the record, here is a list of all the prime years in my lifetime so far:

1973, 1979, 1987, 1993, 1997, 1999, 2003, 2011, 2017

Before 1973 the most recent prime year was 1951, a gap of 22 years, so all that this tells you is that I was born sometime between 1952 and 1972.

I have tried to find some kind of pattern in the sequence above but there are no predictable patterns in prime numbers. Interestingly (and no doubt coincidentally) there have been big election results in some of these years: 1951, 1979 and 1997 all saw big shifts in power in the UK. After their landslide victory in 1945 Labour lost power in 1951. Margaret Thatcher came to power in 1979, Tony Blair in 1997.

The sequence of prime years for the rest of the century is as follows:

2027, 2029, 2039, 2053, 2063, 2069, 2081, 2083, 2087, 2089, 2099

If there were a pattern behind primes and years, good or bad, then the 2080s could be significant: there are 4 prime numbers between 2080 and 2090, which is as many as there can be in any decade.It has happened on seven previous occasions:


It’s possible that this kind of thing might continue to occupy my mind as the year goes on.

Finally, and because I use this Blog to put all sorts of things that I might want later, here is the Excel formula I used to check whether a number  is prime. (It checks for a number in cell C1.) I found it somewhere on the web and don’t fully understand it. Maybe I will, before 2018 rolls around.

=IF(C1=2,”Prime”,IF(AND(MOD(C1,ROW(INDIRECT(“2:”&ROUNDUP(SQRT(C1),0))))<>0),”Prime”,”Not Prime”))


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