# Log-Likelihood Visualization for Archimedean Copulas

#### Marius Hofert and Martin Mächler

#### 2026-01-13

    require(copula)
    doExtras <- FALSE

## Intro

This vignette visualizes (log) likelihood functions of Archimedean copulas, some of which are numerically challenging to compute. Because of this computational challenge, we also check for equivalence of some of the several computational methods, testing for numerical near-equality using `all.equal(L1, L2)`.

## Auxiliary functions

We start by defining the following auxiliary functions.

    ##' @title [m]inus Log-Likelihood for Archimedean Copulas ("fast version")
    ##' @param theta parameter (length 1 for our current families)
    ##' @param acop Archimedean copula (of class "acopula")
    ##' @param u data matrix n x d
    ##' @param n.MC if > 0 MC is applied with sample size equal to n.MC; otherwise,
    ##'        the exact formula is used
    ##' @param ... potential further arguments, passed to <acop> @dacopula()
    ##' @return negative log-likelihood
    ##' @author Martin Maechler (Marius originally)
    mLogL <- function(theta, acop, u, n.MC=0, ...) { # -(log-likelihood)
        -sum(acop@dacopula(u, theta, n.MC=n.MC, log=TRUE, ...))
    }

    ##' @title Plotting the Negative Log-Likelihood for Archimedean Copulas
    ##' @param cop an outer_nacopula (currently with no children)
    ##' @param u n x d  data matrix
    ##' @param xlim x-range for curve() plotting
    ##' @param main title for curve()
    ##' @param XtrArgs a list of further arguments for mLogL()
    ##' @param ... further arguments for curve()
    ##' @return invisible()
    ##' @author Martin Maechler
    curveLogL <- function(cop, u, xlim, main, XtrArgs=list(), ...) {
        unam <- deparse(substitute(u))
        stopifnot(is(cop, "outer_nacopula"), is.list(XtrArgs),
                  (d <- ncol(u)) >= 2, d == dim(cop),
                  length(cop@childCops) == 0# not yet *nested* A.copulas
                  )
        acop <- cop@copula
        th. <- acop@theta # the true theta
        acop <- setTheta(acop, NA) # so it's clear, the true theta is not used below
        if(missing(main)) {
            tau. <- cop@copula@tau(th.)
            main <- substitute("Neg. Log Lik."~ -italic(l)(theta, UU) ~ TXT ~~
                   FUN(theta['*'] == Th) %=>% tau['*'] == Tau,
                   list(UU = unam,
                    TXT= sprintf("; n=%d, d=%d;  A.cop",
                             nrow(u), d),
                    FUN = acop@name,
                    Th = format(th.,digits=3),
                    Tau = format(tau., digits=3)))
        }
        r <- curve(do.call(Vectorize(mLogL, "theta"), c(list(x, acop, u), XtrArgs)),
                   xlim=xlim, main=main,
                   xlab = expression(theta),
                   ylab = substitute(- log(L(theta, u, ~~ COP)), list(COP=acop@name)),
                   ...)
        if(is.finite(th.))
            axis(1, at = th., labels=expression(theta["*"]),
                 lwd=2, col="dark gray", tck = -1/30)
        else warning("non-finite cop@copula@theta = ", th.)
        axis(1, at = initOpt(acop@name),
             labels = FALSE, lwd = 2, col = 2, tck = 1/20)
        invisible(r)
    }

Ensure that we are told about it, if the numerical algorithms choose methods using `Rmpfr` (R package interfacing to multi precision arithmetic MPFR):

    op <- options("copula:verboseUsingRmpfr"=TRUE) # see when "Rmpfr" methods are chosen automatically

## Joe’s family

### Easy case (\(\tau=0.2\))

    n <- 200
    d <- 100
    tau <- 0.2
    theta <- copJoe@iTau(tau)
    cop <- onacopulaL("Joe", list(theta,1:d))
    theta

    ## [1] 1.443824

Here, the three different methods work “the same”:

    set.seed(1)
    U1 <- rnacopula(n,cop)
    enacopula(U1, cop, "mle") # 1.432885 --  fine

    ## [1] 1.432898

    th4 <- 1 + (1:4)/4
    mL.tr <- c(-3558.5, -3734.4, -3299.5, -2505.)
    mLt1 <- sapply(th4, function(th) mLogL(th, cop@copula, U1, method="log.poly")) # default
    mLt2 <- sapply(th4, function(th) mLogL(th, cop@copula, U1, method="log1p"))
    mLt3 <- sapply(th4, function(th) mLogL(th, cop@copula, U1, method="poly"))
    stopifnot(all.equal(mLt1, mL.tr, tolerance=5e-5),
              all.equal(mLt2, mL.tr, tolerance=5e-5),
              all.equal(mLt3, mL.tr, tolerance=5e-5))
    system.time(r1l  <- curveLogL(cop, U1, c(1, 2.5), X=list(method="log.poly")))

    ##    user  system elapsed
    ##   0.383   0.000   0.415

    mtext("all three polyJ() methods on top of each other")
    system.time({
        r1J <- curveLogL(cop, U1, c(1, 2.5), X=list(method="poly"),
                         add=TRUE, col=adjustcolor("red", .4))
        r1m  <- curveLogL(cop, U1, c(1, 2.5), X=list(method="log1p"),
                          add=TRUE, col=adjustcolor("blue",.5))
    })

    ## Warning in plot.xy(xy.coords(x, y), type = type, ...): semi-transparency is not
    ## supported on this device: reported only once per page

    ##    user  system elapsed
    ##   0.843   0.016   0.861

    U2 <- rnacopula(n,cop)
    summary(dCopula(U2, cop)) # => density for the *correct* parameter looks okay

    ##       Min.    1st Qu.     Median       Mean    3rd Qu.       Max.
    ##  0.000e+00  4.900e+01  6.430e+02 2.777e+175  1.932e+04 5.553e+177

    ## hmm: max = 5.5e177
    if(doExtras)
        system.time(r2 <- curveLogL(cop, U2, c(1, 2.5)))
    stopifnot(all.equal(enacopula(U2, cop, "mle"), 1.43992755, tolerance=1e-5),
              all.equal(mLogL(1.8, cop@copula, U2), -4070.1953,tolerance=1e-5)) # (was -Inf)

    U3 <- rnacopula(n,cop)
    (th. <- enacopula(U3, cop, "mle")) # 1.4495

    ## [1] 1.449569

    system.time(r3 <- curveLogL(cop, U3, c(1, 2.5)))

    ##    user  system elapsed
    ##   0.396   0.000   0.423

    axis(1, at = th., label = quote(hat(theta)))

    U4 <- rnacopula(n,cop)
    enacopula(U4, cop, "mle") # 1.4519  (prev. was 2.351 : "completely wrong")

    ## [1] 1.451916

    summary(dCopula(U4, cop)) # ok (had one Inf)

    ##       Min.    1st Qu.     Median       Mean    3rd Qu.       Max.
    ##  0.000e+00  7.500e+01  9.080e+02 1.981e+259  1.434e+04 3.961e+261

    if(doExtras)
        system.time(r4 <- curveLogL(cop, U4, c(1, 2.5)))
    mLogL(2.2351, cop@copula, U4)

    ## [1] -1789.59

    mLogL(1.5,    cop@copula, U4)

    ## [1] -3882.819

    mLogL(1.2,    cop@copula, U4)

    ## [1] -3517.366

    if(doExtras) # each curve takes almost 2 sec
        system.time({
            curveLogL(cop, U4, c(1, 1.01))
            curveLogL(cop, U4, c(1, 1.0001))
            curveLogL(cop, U4, c(1, 1.000001))
        })
    ## --> limit goes *VERY* steeply up to  0
    ## --> theta 1.164 is about the boundary:
    stopifnot(identical(setTheta(cop, 1.164), onacopula(cop@copula, C(1.164, 1:100))),
          all.equal(600.59577,
                cop@copula@dacopula(U4[118,,drop=FALSE],
                        theta=1.164, log = TRUE), tolerance=1e-5)) # was "Inf"

### Harder case (\(d=150\), \(\tau=0.3\))

    n <- 200
    d <- 150
    tau <- 0.3
    (theta <- copJoe@iTau(tau))

    ## [1] 1.772108

    cop <- onacopulaL("Joe",list(theta,1:d))

    set.seed(47)
    U. <- rnacopula(n,cop)
    enacopula(U., cop, "mle") # 1.784578

    ## [1] 1.78459

    system.time(r. <- curveLogL(cop, U., c(1.1, 3)))

    ##    user  system elapsed
    ##   0.481   0.016   0.524

    ## => still looks very good

### Even harder case (\(d=180\), \(\tau=0.4\))

    d <- 180
    tau <- 0.4
    (theta <- copJoe@iTau(tau))

    ## [1] 2.219066

    cop <- onacopulaL("Joe",list(theta,1:d))

    U. <- rnacopula(n,cop)
    enacopula(U., cop, "mle") # 2.217582

    ## [1] 2.217593

    if(doExtras)
    system.time(r. <- curveLogL(cop, U., c(1.1, 4)))
    ## => still looks very good

## Gumbel’s family

### Easy case (\(\tau=0.2\))

    n <- 200
    d <- 50 # smaller 'd' -- so as to not need 'Rmpfr' here
    tau <- 0.2
    (theta <- copGumbel@iTau(tau))

    ## [1] 1.25

    cop <- onacopulaL("Gumbel",list(theta,1:d))

    set.seed(1)
    U1 <- rnacopula(n,cop)
    if(doExtras) {
        U2 <- rnacopula(n,cop)
        U3 <- rnacopula(n,cop)
    }
    enacopula(U1, cop, "mle") # 1.227659 (was 1.241927)

    ## [1] 1.227659

    ##--> Plots with "many" likelihood evaluations
    system.time(r1 <- curveLogL(cop, U1, c(1, 2.1)))

    ##    user  system elapsed
    ##   0.511   0.012   0.549

    if(doExtras) system.time({
        mtext("and two other generated samples")
        r2 <- curveLogL(cop, U2, c(1, 2.1), add=TRUE)
        r3 <- curveLogL(cop, U3, c(1, 2.1), add=TRUE)
    })

### Harder case (\(d=150\), \(\tau=0.6\))

    d <- 150
    tau <- 0.6
    (theta <- copGumbel@iTau(tau))

    ## [1] 2.5

    cG.5 <- onacopulaL("Gumbel",list(theta,1:d))

    set.seed(17)
    U4 <- rnacopula(n,cG.5)
    U5 <- rnacopula(n,cG.5)
    U6 <- rnacopula(n,cG.5)
    if(doExtras) { ## "Rmpfr" is used {2012-06-21}: -- therefore about 18 seconds!
     tol <- if(interactive()) 1e-12 else 1e-8
     print(system.time(
     ee. <- c(enacopula(U4, cG.5, "mle", tol=tol),
              enacopula(U5, cG.5, "mle", tol=tol),
              enacopula(U6, cG.5, "mle", tol=tol))))
    dput(ee.)# in case the following fails
    ## tol=1e-12 Linux nb-mm3 3.2.0-25-generic x86_64 (2012-06-23):
    ##   c(2.47567251789004, 2.48424484287686, 2.50410767129408)
    ##   c(2.475672518,      2.484244763,      2.504107671),
    stopifnot(all.equal(ee., c(2.475672518, 2.484244763, 2.504107671),
                tolerance= max(1e-7, 16*tol)))
    }
    ## --> Plots with "many" likelihood evaluations
    th. <- seq(1, 3, by= 1/4)
    if(doExtras) # "default2012" (polyG default) partly uses Rmpfr here:
    system.time(r4   <- sapply(th., mLogL, acop=cG.5@copula, u=U4))## 25.6 sec
    ## whereas this (polyG method) is very fast {and still ok}:
    system.time(r4.p <- sapply(th., mLogL, acop=cG.5@copula, u=U4, method="pois"))

    ##    user  system elapsed
    ##   0.112   0.012   0.124

    r4. <- c(0, -18375.33, -21948.033, -24294.995, -25775.502,
             -26562.609, -26772.767, -26490.809, -25781.224)
    stopifnot(!doExtras ||
              all.equal(r4,   r4., tolerance = 8e-8),
              all.equal(r4.p, r4., tolerance = 8e-8))
    ## --> use fast method here as well:
    system.time(r5.p <- sapply(th., mLogL, acop=cG.5@copula, u=U5, method="pois"))

    ##    user  system elapsed
    ##   0.114   0.012   0.125

    system.time(r6.p <- sapply(th., mLogL, acop=cG.5@copula, u=U6, method="pois"))

    ##    user  system elapsed
    ##   0.116   0.008   0.125

    if(doExtras) {
        if(FALSE) # for speed analysis, etc
            debug(copula:::polyG)
        mLogL(1.65, cG.5@copula, U4) # -23472.96
    }
    dd <- dCopula(U4, setTheta(cG.5, 1.64), log = TRUE,
                  method = if(doExtras)"default" else "pois")
    summary(dd)

    ##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
    ##   41.59   53.30   81.09  116.91  137.54  707.13

    stopifnot(!is.na(dd), # no NaN's anymore
          40 < range(dd), range(dd) < 710)

## Frank’s family (an already hard case)

    n <- 64
    d <- 5
    tau <- 0.8
    (theta <- copFrank@iTau(tau))

    ## [1] 18.19154

    cop <- onacopulaL("Frank", list(theta,1:d))

    set.seed(11) # these seeds give no problems: 101, 41, 21
    U. <- rnacopula(n,cop)
    cop@copula <- setTheta(cop@copula, NA) # forget the true theta
    system.time(f.ML <- emle(U., cop)); f.ML # --> fine: theta = 18.033, Log-lik = 314.01

    ##    user  system elapsed
    ##   0.015   0.000   0.015

    ##
    ## Call:
    ## bbmle::mle2(minuslogl = nLL, start = start, optimizer = "optimize",
    ##     lower = interval[1], upper = interval[2])
    ##
    ## Coefficients:
    ##   theta
    ## 18.0333
    ##
    ## Log-likelihood: 314.01

    if(doExtras)
        system.time(f.mlMC <- emle(U., cop, n.MC = 1e4)) # with MC
    stopifnot(all.equal(unname(coef(f.ML)), 18.03331, tolerance= 1e-6),
          all.equal(f.ML@min, -314.0143, tolerance=1e-6),
          !doExtras || ## Simulate MLE (= SMLE) is "extra" random,  hmm...
          all.equal(unname(coef(f.mlMC)), 17.8, tolerance= 0.01)
          ##           64-bit ubuntu: 17.817523
          ##         ? 64-bit Mac:    17.741
         )

    cop@copula <- setTheta(cop@copula, theta)
    r. <- curveLogL(cop, U., c(1, 200)) # => now looks fine

    tail(as.data.frame(r.), 15)

    ##          x        y
    ## 87  172.14 2105.690
    ## 88  174.13 2143.642
    ## 89  176.12 2181.637
    ## 90  178.11 2219.675
    ## 91  180.10 2257.754
    ## 92  182.09 2295.874
    ## 93  184.08 2334.034
    ## 94  186.07 2372.232
    ## 95  188.06 2410.468
    ## 96  190.05 2448.742
    ## 97  192.04 2487.051
    ## 98  194.03 2525.396
    ## 99  196.02 2563.776
    ## 100 198.01 2602.189
    ## 101 200.00 2640.636

    stopifnot( is.finite( r.$y ),
          ## and is convex (everywhere):
          diff(r.$y, d=2) > 0)
    options(op) # revert to previous state

### Session information

    ## R version 4.5.2 Patched (2026-01-07 r89285)
    ## Platform: x86_64-pc-linux-gnu
    ## Running under: Debian GNU/Linux 12 (bookworm)
    ##
    ## Matrix products: default
    ## BLAS:   /srv/R/R-patched/build.26-01-08/lib/libRblas.so
    ## LAPACK: /srv/R/R-patched/build.26-01-08/lib/libRlapack.so;  LAPACK version 3.12.1
    ##
    ## attached base packages:
    ##  [1] parallel  grid      stats4    tools     stats     graphics  grDevices
    ##  [8] utils     datasets  methods   base
    ##
    ## other attached packages:
    ## [1] rugarch_1.5-4  gsl_2.1-9      mev_2.1        lattice_0.22-7 bbmle_1.0.25.1
    ## [6] copula_1.1-7
    ##
    ## loaded via a namespace (and not attached):
    ##  [1] gmp_0.7-5                   ks_1.15.1
    ##  [3] sass_0.4.10                 future_1.68.0
    ##  [5] KernSmooth_2.23-26          SkewHyperbolic_0.4-2
    ##  [7] pracma_2.4.6                listenv_0.10.0
    ##  [9] digest_0.6.39               evaluate_1.0.5
    ## [11] nleqslv_3.3.5               mvtnorm_1.3-3
    ## [13] fastmap_1.2.0               jsonlite_2.0.0
    ## [15] Matrix_1.7-4                mclust_6.1.2
    ## [17] truncnorm_1.0-9             stabledist_0.7-2
    ## [19] spd_2.0-1                   codetools_0.2-20
    ## [21] numDeriv_2016.8-1.1         jquerylib_0.1.4
    ## [23] Rdpack_2.6.4                cli_3.6.5
    ## [25] rlang_1.1.7                 rbibutils_2.4
    ## [27] pspline_1.0-21              parallelly_1.46.1
    ## [29] future.apply_1.20.1         cachem_1.1.0
    ## [31] yaml_2.3.12                 otel_0.2.0
    ## [33] polynom_1.4-1               nloptr_2.2.1
    ## [35] bdsmatrix_1.3-7             Runuran_0.41
    ## [37] partitions_1.10-9           globals_0.18.0
    ## [39] R6_2.6.1                    zoo_1.8-15
    ## [41] lifecycle_1.0.5             ADGofTest_0.3
    ## [43] MASS_7.3-65                 Rsolnp_2.0.1
    ## [45] pcaPP_2.0-5                 GeneralizedHyperbolic_0.8-7
    ## [47] bslib_0.9.0                 Rcpp_1.1.1
    ## [49] xfun_0.55                   knitr_1.51
    ## [51] htmltools_0.5.9             rmarkdown_2.30
    ## [53] xts_0.14.1                  fracdiff_1.5-3
    ## [55] compiler_4.5.2              alabama_2023.1.0
    ## [57] DistributionUtils_0.6-2