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 A225871 Number of people required for there to be a 50% probability that at least 4 share a birthday in a year with n days. 4
 4, 6, 7, 9, 10, 11, 12, 13, 15, 16, 17, 18, 18, 19, 20, 21, 22, 23, 24, 25, 25, 26, 27, 28, 28, 29, 30, 31, 32, 32, 33, 34, 34, 35, 36, 37, 37, 38, 39, 39, 40, 41, 41, 42, 43, 43, 44, 45, 45, 46, 46, 47, 48, 48, 49, 50, 50, 51, 51, 52, 53, 53, 54, 54, 55, 56 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(365)=187. For n<1000, the formula a(n) = 2.79 + 2.456*n^0.732 - 1.825/n provides an estimate of a(n) accurate to 0.82. LINKS Christian N. K. Anderson, Table of n, a(n) for n = 1..1000 Christian N. K. Anderson, Table of n, exact probabilities of a(n)-1 and a(n) for n = 1..1000. P. Le Conte, Coincident Birthdays EXAMPLE For a year with 365 days, a(365), the probability that out of 186 people 4 of them share a birthday is 0.495825. The corresponding probability for 187 people is 0.502685, and therefore a(365)=187. PROG (R) library(gmp); #prob of a maximum of exactly k coincident birthdays is BigQ<-function(nday, p, k) { #nday=days in a year; p=people     if(p1, sum(sapply(2:k-1, function(j) BigQ(nday-i, p-k*i, j))), 1)     }     tot } BDaySharedByAtLeast<-function(nday, people, k) {     if(nday<1 | people

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Last modified May 19 19:33 EDT 2019. Contains 323395 sequences. (Running on oeis4.)